Recently I have flashed CM9 and later CM10 preview on my Galaxy S3 I9300 with 64 GB Sandisk SDXC card. The card was formatted in Samsung stock ROM, so naturally, it was formatted as exFAT. CM9 and CM10 reported it as "damaged SD card", but this was expected, since open source AOSP ROMs do not support exFAT. However, when restored to original stock ROM via CWM recovery, the stock ROM was showing "damaged SD card" error, too! Not sure why this has happened - the only info I found was someone mentioning that "CM9 is known to do this on exFAT cards" on this forum.
Connecting the card (via adapter and card reader) to computer didn't help - it showed as a RAW partition, Windows was asking to format it and even reported incorrect size. Trying about 7 different open source and commercial recovery tools did not help. However, if booted to recovery mode, CWM could see the card just perfectly!
So this is how to get the data back:
1. Boot to CWM recovery mode (power off the phone, then hold Volume Up-Home-Power buttons for 5 sec)
2. When in CWM recovery mode, connect phone via USB to computer. The phone should be recognized and drivers installed (if not already).
Note: I had problems with Windows recognizing the phone in recovery mode on some PCs (only 1 out of 3 worked, actually). To solve this, installed drivers from Samsung Galaxy S3 Toolkit (first step after toolkit installation) and problem gone: http://forum.xda-developers.com/show....php?t=1703488
3. Run adb command (from Android SDK platform tools) to ensure that the phone is visible to operating system - it should show serial number of the device and "recovery" next to it:
4. Browse the phone to find the location of sdcard (mine was /sdcard):
Note: when first time connected to phone via "adb.exe shell", running "ls" in /sdcard directory came out empty. Had to "exit" from shell and do "adb.exe shell" again.
5. Pull the files from your device using "adb.exe pull" command:
adb.exe pull /sdcard c:\temp
replace /sdcard with your sdcard directory on the phone (if different) and c:\temp with other folder on your PC, if you wish.
That's it! After files are copied, reboot to stock ROM and reformat the card. You can now copy all data back to it from your PC!
Now I'm trying to find a way to format the card to FAT32. Seems not so straightforward with 64GB cards!