Many Samsung Galaxy S3 i9300 users have been complaining about the lack of a USB mass storage mode on the device and now we've found a way to make it work. UMS is very useful when you want to transfer files between your device and the computer.
Instead of UMS ( USB mass storage ), Samsung Galaxy S3 only features MTP ( media transfer protocol ), which is not as easy to use and fast, so many prefer UMS instead.
It's very easy to enable USB mass storage and it's also a safe procedure, so you will not have to root your device. Your warranty will also not be voided.
Before starting the procedure, make sure to backup the data on your device, because there's an unlikely chance of losing it. In order to backup your data, head over to the following links: AppBAK for Android apps backups | SMS backup & restore | Call Logs backup & restore | APN Settings Backup & Restore.
Now let's head to the procedure itself, which will enable the USB mass storage mode on your Galaxy S3 i9300.
UPDATE: This guide will work only with Samsung Galaxy S3 devices that are rooted, as some users have pointed out. I thought that most Android users are used with applications such as the USB Mass Storage needing BusyBox and / or SuperSU in order to work with Android smartphones.
How to enable USB mass storage mode on Galaxy S3
First you will have to download the apk file here. Then copy the apk to your device and disconnect it from your computer. Then install the app on your device and follow the instructions given by the app.
Once the app is installed, you can open it and select the option you prefer for file transfer, which is very easy. Please let us know if you encounter any problems or if it doesn't work for you. Though we've tested this app on our device and it worked flawlessly, so it should be fine.