Would you like to use your Samsung Galaxy S III as an external USB mass storage hard drive? You can easily do so with the use on a third party software called SGS3 Easy UMS. It appears that Samsung have deviced to remove the USB mass storage option from the SGS3, and uses Media device (MTP) and Camera (PTP) as a way of transferring data. Because the MTP and PTP method is so slow to transfer file it is a pain in the neck to wait for a large file transfer, luckily someone has develop a hack that you could enable your phone as a USB hard drive.
However, this hack only works with an external SD card, so you still won't be able to access the internal phone memory. If you would like access files on the internal phone memory, you will first need to move it to an external SD card.
Note: you will need to root your phone for this app to work. If you don't know how to root your SGS 3, read How to Root Samsung Galaxy S 3 I9300 Model.
1. Download the SGS3 Easy UMS APK file from the xda-developers forum. Scroll down the screen till you see the download section, from there you can select the latest version of the app.
2. Next you will need to install the APK file onto your phone using the AppInstaller. To know how you can do this read:Samsung Galaxy S 2: Install APK File on SD Card.
3. Download and install BusyBox. This make other root apps works.
4. When you want to enable your phone as a hard drive, connect your phone to the PC, then launch the Easy UMS app and tap on the UMS mode button. If you want to toggle back to MTP, just tap on MTP button.
5. Once you have finished using UMS, simply close the app and disconnect your phone from the PC.