[Verizon Samsung Galaxy S III] HMkX2:
This pertains to all CM10.1 builds I think -- I have a thumb on what is causing corruption of (exFat) sd cards. (I know, exFat's not officially supported yet, but CM still should not write to the filesystem blindly!!!)

====Non-FAT card physically inserted====

--------- beginning of /dev/log/main
D/MtpService( 3875): onStorageStateChanged /storage/sdcard1 removed -> unmounted
D/MountService( 656): volume state changed for /storage/sdcard1 (removed -> unmounted)
I/MemorySettings( 5358): Received storage state changed notification that /storage/sdcard1 changed state from removed to unmounted
D/MountService( 656): sendStorageIntent Intent { act=android.intent.action.MEDIA_UNMOUNTED dat=file:///storage/sdcard1 (has extras) } to Us
erHandle{-1}
D/MediaScannerReceiver( 3875): action: android.intent.action.MEDIA_UNMOUNTED path: /storage/sdcard1
I/Vold ( 237): /dev/block/vold/179:97 being considered for volume sdcard
D/Vold ( 237): Volume sdcard state changing 1 (Idle-Unmounted) -> 3 (Checking)
D/MountService( 656): volume state changed for /storage/sdcard1 (unmounted -> checking)
D/MountService( 656): sendStorageIntent Intent { act=android.intent.action.MEDIA_CHECKING dat=file:///storage/sdcard1 (has extras) } to Use
rHandle{-1}
I/MemorySettings( 5358): Received storage state changed notification that /storage/sdcard1 changed state from unmounted to checking
D/MtpService( 3875): onStorageStateChanged /storage/sdcard1 unmounted -> checking
I//system/bin/fsck_msdos( 237): ** /dev/block/vold/179:97
[SIZE="4"]I//system/bin/fsck_msdos( 237): Invalid signature in fsinfo block
[B]I//system/bin/fsck_msdos( 237): FIXED[/B] [COLOR="Red"]<- [B]NOT[/B] fixed, it [B]messed it up!! Signature is perfectly valid for exFat!![/B][/COLOR][/SIZE]
I//system/bin/fsck_msdos( 237): Invalid cluster size: 0
I/logwrapper( 237): /system/bin/fsck_msdos terminated by exit(8)
D/qdoverlay( 243): Set pipe=RGB1 dpy=0; Set pipe=VG0 dpy=0;
E/Vold ( 237): Filesystem check failed (unknown exit code 8)
E/Vold ( 237): /dev/block/vold/179:97 failed FS checks (I/O error)
D/Vold ( 237): Volume sdcard state changing 3 (Checking) -> 1 (Idle-Unmounted)
D/MountService( 656): volume state changed for /storage/sdcard1 (checking -> unmounted)
D/MtpService( 3875): onStorageStateChanged /storage/sdcard1 checking -> unmounted
I/MemorySettings( 5358): Received storage state changed notification that /storage/sdcard1 changed state from checking to unmounted
D/MountService( 656): sendStorageIntent Intent { act=android.intent.action.MEDIA_UNMOUNTED dat=file:///storage/sdcard1 (has extras) } to Us
erHandle{-1}
D/MountService( 656): volume state changed for /storage/sdcard1 (unmounted -> unmountable)
D/MountService( 656): sendStorageIntent Intent { act=android.intent.action.MEDIA_UNMOUNTABLE dat=file:///storage/sdcard1 (has extras) } to
UserHandle{-1}
W/MountService( 656): Insertion mount failed (-4)
E/MP-Decision( 1285): UP Ld:32 Nw:1.990000 Tw:140 rq:5.100000 seq:143.000000
I/MemorySettings( 5358): Received storage state changed notification that /storage/sdcard1 changed state from unmounted to unmountable
D/MtpService( 3875): onStorageStateChanged /storage/sdcard1 unmounted -> unmountable
D/MediaScannerReceiver( 3875): action: android.intent.action.MEDIA_UNMOUNTED path: /storage/sdcard1

I am not familiar enough with CM10.1's code to know exactly what daemon is blindly calling a volatile fsck, but wanted to point out the behavior. At the very least, CM should verify the filesystem is ms_dos before calling fsck_msdos! I'm guessing it is a simple case of missing Filesystem-check error handling.

I'm working on a flashable ZIP that (1) intelligently recognizes exFAT cards, and repairs the damage to the FS sig/header and (2) mounts exFAT cards seamlessly using GPL exFat-fuse. I really hope [Commit 31215 - ext4-fuse support] is reviewed soon -- could use that to knock out fused ext4, ntfs, and exFat file systems all at once!