Getting a Can’t mount /data message when you’re in CWM or TWRP recovery means that your device can’t mount the partition containing all the data for Android to run. Your device won’t boot, but instead you will be stuck in a boot loop. The farthest you will get is the recovery menu, but outside of that you won’t be able to go past the logo screen.
This issue usually appears with users who have installed a custom ROM on their Galaxy S3. Those users have been most probably warned by the developers of the ROM that it might brick their device, and getting this error message basically tells you that your device is bricked.
However, as opposed to a hard brick where your device is pretty much useless and you can’t do anything about it, this is a soft brick and you can fix your device by flashing the stock ROM. You can afterwards try installing a custom one again, but there’s no guarantee that this issue won’t come back. If you’d rather have your phone in a completely usable state, follow the instructions in the method below to flash a stock ROM and continue using it that way.
Samsung’s ODIN is a powerful tool which allows you to save your phone from a number of issues. You can download it from AndroidMTK and install it on your computer, as you will need it. If you’re having issues, install it by right-clicking the setup and choosing Run as administrator. You will also need the stock firmware for your device, which you can get at SamMobile. This is the site with all the available stock ROMs for the Galaxy S3, choose the appropriate one for your country and/or carrier, and download it. Once you’ve downloaded, extract it, to get a *.bin, *.tar or *.tar.md5 file, which are the extensions for the files that ODIN uses. The last thing you will need are the device drivers for seamlessly connecting your phone to your computer. Download them from Samsung’s official website. Once you have everything downloaded, start with the steps below to solve your problem and get your phone back to a working state.
- Boot your device into Download mode. To do this, turn your phone off, and hold the Volume Down, Home and Power Keys When the phone vibrates, let go of the Power key but continue holding the other ones pressed. You will get a warning, press the Volume Up key to continue. Your device is now in Download mode.
- Open ODIN. Look at the log on the left side of ODIN, and connect your device via a USB cable. In the log, you will see an Added! message, along with the ID of the port where your device is connected.
- Depending on the version of ODIN, click on either PDA or Navigate to the stock firmware file and select it, then press Open to add it to ODIN.
- Once you’ve added the file, press Start on ODIN to begin the process. ODIN will now flash the stock firmware on your phone.
NOTE: Do not touch, remove, or otherwise disturb your device until ODIN is done. Failing to do so, may result in a hard bricked device, which is one you can’t fix.
- Depending on the version of ODIN, when it’s done it will either say PASSED or You now know that your device has the stock firmware and will work properly. The device will reboot, and you’re free to unplug it from your PC.
In the odd case that ODIN fails, you can try to update your drivers, restart your computer or run ODIN as administrator.
Having a bricked device can seriously scare you and lead you to believe that it is useless and you need to throw it away. However, this is fortunately a soft brick and you can solve it using the method above to flash the stock ROM. Afterwards, you can always go to a custom ROM if that’s what you need, just be careful and keep in mind that this error may come back if you do so.