Older Android devices support USB mass storage for transferring files back and forth with a computer. Modern Android devices use the MTP or PTP protocols — you can choose which one you prefer.
To select a USB connection protocol, open the Settings app, tap Storage, tap the menu button, and tap USB computer connection. You’ll also see the protocol your device is using as a notification when it’s connected to a computer via USB.
Why Modern Android Devices Don’t Support USB Mass Storage
USB mass storage — also known as “USB mass storage device class,” USB MSC, or UMS — was the way older versions of Android exposed their storage to a computer. When you connected your Android device to your computer, you’d have to specifically tap a “Connect storage to PC” button to make the Android device’s storage accessible to the computer over USB mass storage. When disconnecting it from the computer, you’d have to tap a “Turn off USB storage” button.
USB mass storage is the standard protocol used by flash drives, external hard drives, SD cards, and other USB storage devices. The drive makes itself completely available to the computer, just as if it were an internal drive.
There were problems with the way this worked. Whatever device is accessing the storage needs exclusive access to it. When you connected the storage to the computer, it was disconnected from the Android operating system running on the device. Any files or apps stored on the SD card or USB storage would be unavailable when it was connected to the computer.
System files had to be stored somewhere; they’d never be disconnected from the device, so you ended up with Android devices containing separate /data partitions for “system storage” and /sdcard partitions for “USB storage” on the same internal storage device. Android installed apps and its system files onto /data, while user data was stored on the /sdcard partition.
Because of this hard split, you might end up with too little space for apps and too much space for data, or too little space for apps and too much space for data. You couldn’t resize these partitions without rooting your device — the manufacturer chose the appropriate amount for each partition at the factory.
Because the file system had to be accessible from Windows device, it had to be formatted with the FAT file system. Not only does Microsoft have patents it exerts on FAT, FAT is also an older, slower file system without a modern permission system. Android can now use the modern ext4 file system for all its partitions because they don’t need to be directly readable by Windows.
Connecting an Android phone or tablet to a computer as a standard USB storage device is convenient, but there are too many downsides. The craziness had to stop, so modern Android devices use different USB connection protocols.
MTP – Media Device
MTP stands for “Media Transfer Protocol.” When Android uses this protocol, it appears to the computer as a “media device.” The media transfer protocol was widely promoted as a standardized protocol for transferring audio files to digital music players using Windows Media Player and similar applications. It was designed to allow other media player companies to compete with Apple’s iPod and iTunes.
This protocol works very differently from USB mass storage. Rather than exposing your Android device’s raw file system to Windows, MTP operates at the file level. Your Android device doesn’t expose its entire storage device to Windows. Instead, when you connect a device to your computer, the computer queries the device and the device responds with a list of files and directories it offers. The computer can download a file — it will request the file from the device, and the device will send the file over the connection. If a computer wants to upload a file, it sends the file to the device and the device chooses to save it. When you delete a file, your computer sends a signal to the device saying, “please delete this file,” and the device can delete it.
Android can choose the files it presents to you, and hide system files so you can’t see or modify them. If you attempt to delete or edit a file that can’t be modified, the device will refuse the request and you’ll see an error message.
Your computer doesn’t need exclusive access to the storage device, so there’s no need to connect the storage, disconnect it, or have separate partitions for different types of data. Android can also use ext4 or any other file system it wants — Windows doesn’t have to understand the file system, only Android does.
In practice, MTP functions a lot like USB mass storage. For example, an MTP device shows up in Windows Explorer so you can browse and transfer files. Linux also include offers for MTP devices via libmtp, which is generally included with popular desktop Linux distributions. MTP devices should appear on your Linux desktop’s file manager, too.
Apple’s Mac OS X is a holdout — it doesn’t include MTP support at all. Apple’s iPod, iPhone, and iPad use their own proprietary syncing protocol along with iTunes, so why would they want to support a competing protocol?
Google provides an Android File Transfer application for Mac OS X. This application is just a simple MTP client, so it will work for transferring files back and forth on a Mac. Google doesn’t provide this application for other operating system because they include MTP support.
PTP – Digital Camera
PTP stands for “Picture Transfer Protocol.” When Android uses this protocol, it appears to the computer as a digital camera.
MTP is actually based on PTP, but adds more features, or “extensions.” PTP works similarly to MTP, and is commonly used by digital cameras. Any software program that supports grabbing photos from a digital camera will support grabbing photos from an Android phone when you select the PTP mode. PTP was designed to be a standard protocol for communicating with digital cameras.
In this mode, your Android device will work with digital camera applications that support PTP but not MTP. Apple’s Mac OS X does support PTP, so you can use PTP mode to transfer photos from an Android device to a Mac over a USB connection without any special software.
If you have an older Android device, you may be forced to use USB mass storage. On a modern Android device, you have a choice between MTP and PTP — you should use MTP unless you have software that only supports PTP.
If your device has a removable SD card, you can remove the SD card and insert it directly into your computer’s SD card slot. The SD card will be available to your computer as a storage device, so you can access all the files on it, run file-recovery software, and do anything else you can’t do with MTP.
Image Credit: Vegetando on Flickr