How-To Geek

What Are the Windows A: and B: Drives Used For?


The C: drive is the default installation location for Windows, if you have a CD/DVD drive on your machine it’s likely the D: drive, and any additional drives fall in line after that. What about the A: and B: drives?

Image by Michael Holley.

Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-drive grouping of Q&A web sites.

The Question

If you’re a geek of a certain vintage—we won’t start naming years—the answer to this question is patently obvious to you. For younger geeks, however, the A: and B: drive have always been mysteriously unaccounted for on their computers.

SuperUser reader Linker3000 poses the question:

In Windows you have a C: drive. Everything labeled beyond that is with the following letter.

So your second drive is D:, your DVD is E: and if you put in a USB stick it becomes F: and the following drive G:. And so on and so forth.

But then, what and where are A: and B:?

What and where, indeed? Thankfully we have some seasoned geeks to answer the query.

The Answers


Image by AJ Batac.

Veteran geek Adam Davis offers an in-depth look at the missing drive letters:

The early CP/M and IBM PC style computers had no hard drive. You had one floppy drive, and that was it. Unless you spent another $1k or so on a second floppy drive, then your system was smokin’! If you only had one drive it was common to boot from one disk, put in the other disk with your programs and data, then run the program. Once the program finished, the computer would request that you reinsert the boot disk so you could use the command line again. Copying data from one disk to the other was a series of “Please insert source disk into drive A:… Please insert destination disk into drive A:… Please insert source disk into drive A:… . . .”

By the time hard drives became cheap, the “expensive” computers typically had two floppy drives (one to boot and run common programs, one to save data and run specific programs). And so it was common for the motherboard hardware to support two floppy drives at fixed system addresses. Since it was built into the hardware, it was thought that building the same requirement into the OS was acceptable, and any hard drives added to the machine would start with disk C: and so forth.

During the transition from 5.25″ disks (which were actually, physically floppy) to 3.5″ disks (which were encased in a harder plastic shell) it was common to have both drives in one system, and again it was supported on the motherboard with hardware, and in the OS at fixed addresses. As very few systems ran out of drive letters, it was not thought to be important to consider making those drives re-assignable in the OS until much later when drives were abstracted along with addresses due to the plug’n’play standard.

A lot of software was developed since that time, and unfortunately much of it expected to see long-term storage on the C: drive. This includes the BIOS software that boots the computer. You can still attach two floppy drives, boot into DOS 6.1, and use it as you would have in the early 90’s, with floppy drives A and B.

So largely the reason for starting the hard drive at C is for backwards compatibility. While the OS has abstracted data storage to some degree, it still treats A and B differently, in such a way that allows them to be removed from the system without altering the OS, caching them differently, and due to early viruses treating their boot sector with more caution than the hard drive’s boot sector.

SuperUser contributor Nick chimes in with an interesting anecdote building off of the third paragraph of Adam’s answer dealing with letter assignments:

Less an answer, more of an anecdote. In this Microsoft article, it says:

“You can assign the letters C through Z to each drive on your computer. A and B are usually reserved for floppy disk drives, but if your computer does not have floppy disk drives, you can assign A and B to volumes.”

So when I built a new computer recently with two internal drives, one for the OS and one for data, I thought, hey!, I’ll make my data drive “A”. I felt all rebellious until I discovered that Windows will not index drives lettered A or B. :(

Took me quite a while to figure out what the problem was, but I found some other people who suffered the same issue when they used A or B for a [primary] drive. As soon as I assigned that drive a different letter, windows indexed the drive. So much for being rebellious.

So much for being rebellious indeed—if you want to live on the edge you can assign a data drive to A: and B:, but not a boot drive.

Have something to add to the explanation? Sound off in the the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.

Jason Fitzpatrick is warranty-voiding DIYer and all around geek. When he's not documenting mods and hacks he's doing his best to make sure a generation of college students graduate knowing they should put their pants on one leg at a time and go on to greatness, just like Bruce Dickinson. You can follow him on if you'd like.

  • Published 08/28/12

Comments (64)

  1. Rich

    Very interesting post… so that’s the reason I can’t install Windows 8 en drive A:

  2. r

    they should release Win8 exclusively on just 5.25″.
    that would shake things up in the industry –help gain consumer confidence and all.

  3. TallPaul

    Really? I’m 20 and I know what the A: drive was for. I assumed previously that some people might have a second disk drive and that is what B: was reserved for.

  4. sj

    omg. I’m old.

  5. Grant

    Why can’t it be for my USB flash drives now? They are the modern floppy replacement.

    Actually, mine is /mnt/USBDRIVE/

  6. Matthew

    I’ve used 3.5″ and 5.25″, and I’m only 21. On my laptop, I’ve got a dead optical drive mapped to A: and the onboard SD card reader mapped to B:

    Of course, don’t have to worry about drive letters on Ubuntu.

  7. nidzo

    I feel old. I’m 16 and I have used floppies.

  8. williamknight57

    I use A for my genealogy flashdrive, every works fine, unless I plug it in after open certain program then it come up labeled as floppie drive A, but still works fine

  9. SatoMew

    19 here and have used 3.5” floppies when I was younger and mostly back in the Windows 95, 98 and 98 SE days.

  10. Mike

    Man, this article makes me feel old! Reminds me of my Tandy days! Thought I was all that when I bought a brand new 086, cutting edge tractor fed printer, Hayes 56K external modem and a list of 800 number Wildcat boards to choose from!! DOS games where the ticket!! Oh the joy of finding and connecting to a free board!! LOL!! I’ll tell you a little secret…I had a PC up to a couple of years ago that had a 5.25″ drive in it (by design). I’ve got a box of 5.25″ floppy’s with all kinds of stuff on them. Old habits are hard to break. LOL!!

  11. Frank

    when I bought my wow double-floppy PC in 1988 or so, A: was for the operating system 5.25″ floppy, B: was for the data/documents/personal files 5.25″ 360KB floppy later 720KB.

    I do remember lots of floppy swapping to copy files – and the sounds – slide, turn lock handle, press Enter or somesuch, drive starts up – bzzt, bzzt, message appears on screen, insert other floppy, turn handle, pull out, put on stack of slippery suckers, find/insert correct one, hopefully without bending/scratching it, repeat many times.

    when 3.25″ hard ‘floppies’ came out in 1.2MB – wow – humungous ! and neat little boxes to store them in !

    seemed like it was maybe $1000 to buy the first hard disks for PCs – what were they – 2/4/8 MB or something ?

    now my students lose 16GB USB thumbdrives and think nothing of it – they only cost ten bucks – meh …
    you could lose a 32GB micro-SD card through a small hole in your pocket !

  12. CrazyManNo9

    Original floppies were 8 inches. 5.25″ floppies were an upgrade.

  13. Ashiq

    I remember playing games from a floppy drive. I used to have a large collection of floppy disks.

    So I already knew what drive A was but did not have a clue about drive B

  14. Phil

    I don’t ever recall second floppy drives selling for $1000 – maybe in some absurd “retail” computer place, but not actual real-world prices. The early SA400 Shugart floppy drives went for around $500 in the mid 1970s. When the Apple II’s Disk II came out the drive by itself was $495, with the controller card (which could support two drives) for $100.

    I recently reinstalled an old 5.25″ drive into one of my backup PCs. It now has both 3.5″ and 5.25″ drives on it – just in case. Once in a rare moon I’ll come across a floppy disk which has data I need to access and it’s useful to have a computer with those drives. I helped out a friend a couple of months ago by transferring the contents of a 5.25″ floppy on to a flashdrive. Backwards compatibility is useful.

  15. UltimatePSV

    I’m almost 18, and I remember using the A: drive a couple times as a little kid; I just pulled all the floppy disk data from some old ones a few months ago for my dad, actually, since we don’t want to keep using them. As for the B: drive, I never had one.

    I always thought A: was for 3.5″ floppies and B: was for 5.25″… I guess that doesn’t make much sense now that I think about it…

  16. kura

    so much for being rebellious lol

  17. Suicidal Idiot

    At work, I’m bumping the last Windows PC’s to 7 and just realized two days ago that almost everybody has A and B available. I’ve started mapping A: to a network share containing our Act! database.

    Anybody have fun/irreverent ideas for general mapping to B?

  18. David Aris-Sutton

    Being 37 I knew the answer to this immediately but then I realized that I couldn’t remember whether my old Acorn Electron assigned an address to the drive, which was a tape cassette recorder.
    The hours i used to spend waiting for programs to load from those c60 tapes

  19. Cryphakon

    I feel old that I know this and I’m only 16, still have a laserdisc player somewhere in the basment.

  20. Lost My Java (and I don't care)...

    I wonder if the newer UEFI (ooh-fee) will even support floppy drives. I know it’s possible to still use a USB attached floppy drive which will want to be either A: or B: And I know of only one Linux distro that is actually designed to boot and run entirely from a floppy disk – mu linux. There’s also the PLoP loader that can also boot from floppy which I’ve found very handy. So there may still be a use for these things after all.

    Seems like the floppy drive connectors haven’t exactly disappeared from all motherboards either. It’s like a technological wart on the entire industry. These things have been around since before ISA (a.k.a. “stone age” computers). In fact, my local supplier still sells 3.5″ HD floppy disks and drives! I just wish I had more of a use for them especially now that a USB flash drive can cram like a billion times the capacity for almost the same money as a box of “HD” floppy disks (that HD label still cracks me up).

  21. NSDCars5

    Hey! I’m 11, and I’ve used floppies, and know what A: and B: are for. But I did install Windows on A: and B:. Windows XP on A:, Windows 7 on B:, and Windows 8 RP on C:. I usually booted to A:. :/

  22. Kevin

    The “C” stands for “Christ” as in “Christ, my hard drive just failed and I don’t have a backup”.

  23. malachipclover

    I’m 13 and remember using floppies.

  24. KMFDMKid2000

    I’m 12 and what is this?

    Memes aside, floppies were the original pirate medium. Anyone remember “Don’t Copy That Floppy”? Go look it up on YouTube, it’s like the Fresh Prince threw up on a PSA.

  25. Lantiis

    Awesome explanations! I laughed at both of them. I remember A: and B: … some even recently as we have some departments here who still used floppy disks through last year (to our dismay in IT). So we had to find interesting ways of continuing to support them until they were able to back up their data in a more modern way (or rather, until IT backed their data up for them because they couldn’t figure out how to save the data to the network drives…). Ugh.

  26. truckdriverfritz

    getting a kick out of all the “I feel old” comments… I used 8″ !! floppies when I first started at Zenith Data (remember them?) my engineer would “give” me the latest thing to play with/learn… remember him giving me the latest fastest hard drive saying, “I’m only giving you a 5mg… You’ll get lost in a 10…”

  27. Jason

    @Suicidal Idiot I’ve previously had B mapped to a [B]ackup partition.

  28. charli

    i think i still have a case of floppy disks laying around somewhere.

  29. dcj2

    l still occasionally use the old “subst” command to assign a: and/or b: to folders on the hard drive. Very handy with long paths, and to my eye easier to read in a script than a %xyz% variable.

    Reading the article also reminded me of the old disk notchers. Back in the day you could buy single-sided (SSFD) or double-sided floppies (DSFD). All disks were manufactured as double-sided, but if one side failed quality control, it was sold as a single. On a DSFD, you’d take the disk out and flip it over to use the other side. Naturally, the DSFD’s were much more expensive, but if you only needed a scratch disk (or didn’t care if the data was lost), you could cut a little notch in the side of the sleeve to convert a single to a double. You could buy a special tool to do it for about $20, but most of us just used a razor blade. Ahh, the good old days!

  30. wwasion

    I’ve used B to map my backup drive for years.

  31. Allen

    I’m only 17 and i knew exactly what those drives were for… I still use floppies on occasions, usually to flash and old computer’s bios or to start a network boot on a card that doesn’t support it.

  32. Richard

    Without wanting to appear really past it, I remember using paper tape readers, that was modern because really old people had used card readers before us yung uns started. The transition to programs on cassettes was mind blowing, we dreamt of floppies.

  33. Diptiman

    Feeling sad with 3 box’s of floppy (windows 3.1) :(

  34. zepe

    I use the B drive designation for my internal backup drive. When I set up or build a computer, I set up my temporary internal backup drive as drive B in Disk Management.


  35. Gary Marc

    I have to laugh just because I must be older than most of you that have responded. I still have my EIGHT INCH drives from my early days with CPM and DOS and my Z80 CPU system.

    I am thinking of starting a Museum of antique computer equipment and software. Donations welcome!

  36. Paul Kowalski

    I still remember seeing 8 inch floppy disks and still have an 8 inch floppy disk carrier/file box.
    I also have a “notch cutter” so I could use the other side of 5.25 inch floppies before DSDD.

  37. john3347

    I don’t remember the $1000 additional cost for a second floppy drive (I think this is the number that the geek shorthand,$1K, represents). My first personal computer, an Amstrad with an 8088 processor, had two 5 1/4″ floppies and cost around $1000 or $1200. This was mid80s. You normally put the operating system in one drive and the application in the other drive. I forget which one you removed to put the data disk in to save your work.

  38. jthelw

    Those were the bad old days!

  39. lesle

    I assign B: to my Backup hdd (and I: to my system Image hdd). I have assigned A: to my W7 system reserve partition, I no longer do, no point to it.

  40. Jim

    I remember using 256KB 8-Inch floppies on Intel “Blue Boxes”

    Old? Who’s old?

  41. chanks

    CrazyManNo9 is correct, 8″ floppies were first, single sided and double sided. Single sided were 512K and double sided were 1MB yep, 1 megabite. The first hard drives were 8″ also, 20mb.

  42. shinigamibob

    So what happens when you hit drive Z: and have to add another drive? Does Windows just ignore it or does it start combining letters, like AA or AB?

  43. FichenDich

    “Back in the days before the ferns & dinosaurs turned into oil, we used kay-set tapes for storage and were damned happy to have them !”

    “It used to burn my britches when my girl friend’s father used to derisively snort, ‘Pshaw ! Punch cards will be around long after them thar new fangeled kay-sets are relegated to the ash bins of History !’ “

  44. Mike

    What are we in to the PC computer era some thirty or forty years now? I am always stunned by this industry’s ability to drag the chains of the past around behind it like the chains of Marley. How about some real groung breaking on the part of Microsoft and Apple. Clean up all this old junk rather than screwing with the GUI like Metro. What I would like to see is this: A product or God forbid a revision of Windows etc. that would “see” any type of disk attached to the PC in a uniform way in the GUI somewhat like the layout for hard drives in Windows 7 and a right click or mouse click operation that will change for each type of drive to give you access to each type of device and its functions.

    For instance, using the old taskbar GUI model, an icon that sits on the taskbar. You click on the icon and a window opens with icons for each device i.e. a floppy for any old floppy devices, a CD icon for each CD drive, a hard drive for each hard drive etc. Clicking on the hard drive icon would show all the partitions (even linux partitions) on the drive. A way to click on the CD/DVD drives that would allow you to physically open AND close the CD draw as well as the content. A uniform model that would tie the driver and the model together. This could also be done with phones, tablets etc. The model would also work on a desktop or laptop so that you could see your phone or tablet etc. and since many things are either ethernet or wifi connected these devices could also be added to this GUI as well.

    Microsoft has this death grip on the hardware makers that they could use to enforce an API model to support this if they would actually focus on making the system better instead of adding the junk and trying to lure everyone into the cloud.

  45. Bud Vitoff

    Old? Who’s old? I will never forget the customer who used a CDC 160A with punched paper tape I/O and a Friden Flexowriter as a printer.

    Much later, I worked with a TRS-80 Model II with four 8″ drives: A, B, C, and D.

  46. Willy

    Hmmmm, I wonder if the a: and b: drives could be somehow linked in the eprom setup to two of the USB ports on a computer and reserved for the first two thumb drives? Just thinkin’ out loud.

  47. vitally

    Do you also remember the square punch to make the floppies write-proof ?

  48. spike

    @Suicidal Idiot: I’ve used B: for Backup for ages.. never a problem :) I use A for my main data storage on my main laptop. Guess I’m rebellious :)

  49. Tim

    Guys i still have nc machines running stiffys every day i still have about 10 spare drives i even have two new floppy drives .customers dont just change because its new .i still supply stiffys to them but the end is in site .

  50. Edgar

    That question looked so stupid to me that I have one conclusion: I’m very old! lol

  51. jackblue

    I’ve got a USB 3.5″ floppy drive that uses drive A: when I plug it in. It’s fun to look back at some of the stuff I used to use and thanks to D-Fend Reloaded I can do that. As a power user (back in the day), I had an Amstrad 640 which I called “Jack’s Flyer” – because it had 2 yes TWO 5.25″ floppies!

  52. LadyFitzgerald

    You kids make me laugh. I still have a bunch of 5.25″ and 3.5″ floppies (and a Commodore C64c with the drives–one 5.25″ and two 3.5″–to use the floppies in). I can remember using punch cards at work and saw them being used in the mainframe (my last little netbook had more computing power than that mainframe, which took up a large room). It was years before my department rated mainframe terminals. We tracked stock activity on large cards (roughly 24″ square) that had the data manually entered on it from a daily printout (about a two inch thick stack of paper) using a huge thermal transfer machine (that and updating the card file was seriously BORING!).

  53. LadyFitzgerald

    Now that I think about, I probably still have my 5.25″ notch cutter still knocking about. And the little pieces of tape used to un-write-protect the disk.

  54. Pat

    Floppies? Hard drives? What’s with all of that? Computers are those things hidden in the basement of an obscure building that assign you classes and classrooms. They’re the soulless monsters that assign you to an 0800 class every day with an 0900 class 2 km away on the other side of campus. They need their own power plant to operate and 100 tonnes of ice a day to keep them cool. You mean they have OTHER computers?

  55. Kelly Lucas

    This brings back fond memories of my Tandy 1000, all I did was play King’s Quest and a couple of other games and write and print letters to family & friends instead of actually writing them by hand lol, I wonder what computers and USB drives will be like in 5 years.

  56. Jack in TN

    My first home computer, Altair 8800, I eventually had 2 8″, double sided, double density drives, and a HUGE 5M 5.25″ 3″ tall hard drive. Also 64K of memory (1K was ROM, 1K was a memory mapped video display, and the rest was RAM). I upgraded the Intel 8008 2MHz CPU. Eventually it was upgraded to a 4MHz Zilog Z80 processor. The paper tape reader, AI Cybernetics Speech Synthesizer, DCHayes 300 baud modem board, SASI disk interface board (for the 5M Shugart hard drive), a home made interrupt controller board, and a Processor Technology 2P+S I/O card, was a smoking hot system for the day! To drive it all, a friend and I rolled our own replacement power supply to provide +-12VDC, and a lot of +5VDC, and a little -5VDC. All with big capacitors and a couple of big transformers (the external power supply cabinet was the same size as the Altair and several times heavier. The Altair actually sat on-top of the power supply when in use! The video displayed on a modified B/W TV set (modified to give me a non-isolated composite video input, basically inserted a signal after the tuner, but before the first RF stage in the display section).

    Ahh, hacking in the ’70&’80s. Those days were great. These days would be better if the arthritis wasn’t kicking in so bad!

  57. JB

    You lie. You’re not 11 and you cannot boot a HD mounted on A:

  58. Andy Johnson

    My first CP/M PC came with 8 inch floppies that soon became obsolete, as everything changed to 5 1/4″ floppies. Not long afterwords the 3 1/2 inch replaced everything. I used to save all he data to the new formats, so I still have a bunch of the 3.5″ disks around.

  59. Torch

    Before 5.25″ floppys, there were 8″ floppys.

  60. oldman river

    I have a zip drive in a very old machine. It was Iomega 100 mb formatted for IBM compatibles (disk).
    The drive is in a machine (prior to windows 95). Would this be qualified as a floppie? I used these
    disks to do backups. I do not have the device drivers, but I do have a bunch of old floppies.

  61. boris

    bought my first pc in 1991, used all the above lol – loved it! guess im old now

  62. Jack Meili

    My first computer was the IBM Portable Computer. It had 2 floppy drives and NO hard drive. I still have it – someplace. I never used it much. It wasn’t convenient to use. I started working for IBM in 1961. I was one of several on the team which considered the creation of a floppy drive using disks smaller than 8 inches. I don’t recall how 5.25 inches was settled on. Since San Jose was the manufacturing site for all IBM disks, it was logical for the decisions to be made here. My recollection is that all 5.25 and 3.5 inch disks were manufactured at IBM Rochester, MN as long as the patents were valid.

  63. Israel

    Are there floppy disks still today? Dang, I thought they didn’t exist anymore!

  64. clamo

    hm….I already knew all this and I know more than what has been posted, not going to bother saying anything as some A-hole and so called IT pro will tell me to stop spreading misinformation. I still use 2.5″ floppy’s as some of my programs will not work on cd’s.

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