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Why, Exactly, Do You Need to Safely Eject USB Media?

Windows gets so mad when you don’t safely eject USB media, but does it really matter? What’s the worst that could happen if you never safely ejected your USB drive and other flash-based media?

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

The Question

SuperUser reader Simon is really curious what fate might befall him if he never safely ejects his media:

Quite often when I’m in a rush, I automatically pull out a USB pendrive or USB cable from an external hard drive from my desktop PC or laptop, without right-clicking on the safely remove icon in the system tray and unplugging via this route. So far nothing untoward has happened every time I have “been in a rush”.

What is the rationale behind right clicking on the safely remove icon and can I really lose information on USB media if this is not carried out ?

Is the [probability] of losing such information greatly increased, if the USB media is still flashing at the time of pulling it out of the computer (as opposed to non-flashing) ?

How much of a gamble is Simon taking?

The Answer

SuperUser contributor Dave Rook explains:

Yes it can, it’s about what happens if you remove the device when it’s in use (reading or writing):

When you plug in a USB drive, you give your PC free rein to write and read data from it; some of which is cached.

Caching occurs by not writing information immediately to the USB device, and instead keeping it in your PC’s memory (RAM). If you were to yank the USB drive out of your PC before this information is written, or while its being written, you’ll end up with a corrupted file.

However, Windows automatically disables caching on USB devices, unless you specifically say that you want it enabled. For the most part you don’t have to click the ‘Safely Remove Hardware’ button, if you aren’t writing or reading anything from the device.

Its there simply as an extra level of security preventing you from destroying your own files.

Doing so causes the files to close “gracefully”, preserving data, pointers and file size indicators. When writing to disk the computer doesn’t always “flush” a buffer and only part of the data may have been written. Using the proper procedure will assure that the data and pointers are in good shape.

Source

MSalters offers a sobering insight:

A second reason is that flash drives need to have stable power for ~0.25 seconds after a write command. This is a fundamental physical problem, due random factors some writes may leave a logical 1 bit in a electrical 0.72 state. The fix is easy: just rewrite the bit, perhaps even a few times. Eventually it will stick.

If you’re really unlucky, the bit falling over will be in a filesystem table and corrupt e.g. an entire directory.

In other words, it’s not worth gambling with which bit might or might not be corrupted: it might be a temporary file in a portable application’s cache or it might be, as MSalters points out, a critical system file.

For more information about safe media ejection, check out: HTG Explains: Do You Really Need to Safely Remove USB Sticks?


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 10/10/13

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