Android USB Connections: MTP, PTP, and USB Mass Storage Explained

File Transfer Protocols on Android Devices

Historically, older Android devices utilized USB mass storage as a method for file transfer between the device and a computer.

However, contemporary Android devices now employ either the MTP (Media Transfer Protocol) or PTP (Picture Transfer Protocol) for this purpose.

Selecting Your Preferred Protocol

Users have the ability to choose their preferred USB connection protocol. This selection can be made through the device's settings menu.

To access these settings, open the Settings application, then navigate to the Storage section.

Within Storage, locate and tap the menu button, followed by selecting USB computer connection.

Protocol Status Notification

The currently active USB protocol is also displayed as a notification whenever the Android device is connected to a computer via USB.

This provides immediate feedback regarding the connection type being utilized for file transfer.

The Discontinuation of USB Mass Storage on Modern Android Devices

Older Android versions utilized USB mass storage – also referred to as USB MSC or UMS – as the method for exposing storage to a computer. Connecting an Android device previously required a deliberate tap on a "Connect storage to PC" option. Similarly, disconnecting necessitated selecting "Turn off USB storage."

USB mass storage represents the standard protocol employed by flash drives, external hard drives, and SD cards. The drive presents itself as fully accessible to the computer, functioning much like an internal drive would.

However, this approach presented several challenges. Exclusive access was required by whichever device was accessing the storage. When a connection to a computer was established, the Android operating system on the device lost access to the storage.

Consequently, any files or applications residing on the SD card or USB storage became unavailable during the computer connection. System files needed to remain consistently accessible, leading to a partitioning scheme on Android devices.

Android devices were configured with distinct /data partitions for system storage and /sdcard partitions for USB storage, often residing on the same physical storage. Applications and system files were installed on /data, while user data was stored on /sdcard.

This rigid partitioning could result in imbalances – insufficient space for applications or an excess of space for data, and vice versa. Resizing these partitions demanded rooting the device, a process controlled by the manufacturer during factory configuration.

Related: Why Do Removable Drives Still Use FAT32 or exFAT Instead of NTFS?

To ensure compatibility with Windows systems, the file system was typically formatted with FAT. Beyond patent restrictions held by Microsoft, FAT is an older file system lacking the advanced features and speed of modern alternatives. Android now leverages the ext4 file system for its partitions, as direct Windows readability is no longer a constraint.

While convenient, connecting an Android phone or tablet as a standard USB storage device introduced too many drawbacks. Therefore, contemporary Android devices now employ alternative USB connection protocols to overcome these limitations.

MTP – Media Device Explained

MTP, which stands for Media Transfer Protocol, defines how an Android device is recognized by a computer as a media device. Initially, this protocol was heavily promoted as a standardized method for transferring audio files to digital music players, particularly when utilizing Windows Media Player and similar software.

The intention behind MTP was to foster competition with Apple’s iPod and iTunes ecosystem by providing an open standard for media transfer.

How MTP Differs from USB Mass Storage

The operation of MTP is fundamentally different from traditional USB mass storage. Instead of granting Windows direct access to the Android device’s entire file system, MTP functions at the file level.

When a connection is established, the computer doesn’t see the raw storage. Rather, it requests a list of available files and directories from the Android device, which then responds accordingly. File transfers involve requests and responses – a computer requests a file, and the device sends it.

Similarly, uploading a file involves the computer sending the file to the device, which then determines where to save it. Deletion requests are also handled this way; the computer signals the device to delete a specific file.

File Management and Security

Android maintains control over which files are presented to the computer, allowing it to conceal system files from view and prevent unauthorized modification. Attempts to alter restricted files will be denied, resulting in an error message.

Because of this architecture, exclusive access to the storage isn’t required. This eliminates the need for disconnecting and reconnecting storage or creating separate partitions for different data types.

Android is free to utilize file systems like ext4, as Windows only needs to interact with the MTP protocol, not the underlying file system itself.

Practical Usage and Compatibility

In many ways, MTP behaves similarly to USB mass storage for the user. Devices utilizing MTP are typically visible in Windows Explorer, enabling file browsing and transfer.

Linux systems also offer support for MTP devices through the libmtp library, commonly included in popular distributions. Consequently, MTP devices should be accessible within most Linux desktop file managers.

However, Apple’s macOS remains an exception, lacking native MTP support. Apple prefers to utilize its proprietary syncing protocol with iTunes for its iPod, iPhone, and iPad devices.

MTP on macOS

For macOS users, Google provides the Android File Transfer application. This application functions as a dedicated MTP client, facilitating file transfers between a Mac and an Android device.

Google does not offer this application for other operating systems because they already incorporate MTP support.

PTP - Digital Camera Protocol

PTP, which is an acronym for "Picture Transfer Protocol," defines a method for transferring images. When utilized by an Android device, it presents itself to a computer as a standard digital camera.

While MTP builds upon the foundation of PTP, it incorporates additional functionalities and extensions. The operational principles of PTP closely resemble those of MTP, and it is frequently employed by digital cameras.

Any software capable of retrieving photographs from a digital camera will also be able to access photos from an Android phone when PTP mode is selected. PTP was originally conceived as a standardized communication protocol specifically for digital cameras.

Compatibility and Usage

Selecting this mode allows your Android device to function seamlessly with digital camera applications that recognize PTP, but may not support MTP. Notably, Apple’s Mac OS X operating system offers native PTP support.

This enables photo transfers from an Android device to a Mac via a USB connection without the need for installing any specialized software.