I recently had a Micro SD card that stopped being able to be read after applying a ‘kitkat’ patch to allow my rooted phone to use the card from other applications. It was more likely the patch itself didn’t cause the problem, but a large file I was moving corrupted the file system.
After rebooting my phone I got the dreaded SD Card with Exclamation point ‘Unreadable Card’ Instead of formatting it I decided to attempt to recover it.
It was unreadable from either Ubuntu nor Windows,
In windows it didn’t provide very useful information, but in attempting to mount it in Ubuntu I at least got a real error ‘Mount Failure, Could not read Superblock’ which gave me a hint on what was needed to fix it.
I could have tried to repair it with GPARTED in Ubuntu but I didn’t have it installed at the moment so instead I booted my laptop into Windows 8.1 and from an Administrative Command Prompt (Launch CMD as administrator)
Fortunately, the SD card partition is FAT32 so I attempted to fix it:
CHKDSK D: /F /V /R /X
D: was the SD card with the problem.
This might also work with other removable drive types like USB Thumb drives.
A word of warning, Copy all of the files off of the card before using it again to avoid losing anything you might want off the card.
Ultimately it would be a good idea to copy all the data off, reformat the card, and copy it back.