Broken SONY SD Card, formatting, "shredding", adding/deleting files to no avail
You can always try:

1)Format/repartition SD card using a PC and ensure that everything in it has been deleted.

2)Wipe your phone from TWRP (data,cache,dalvik cache,system).

3)Reinstall CM10 from scratch.

4)Insert SD card.

5)Link2SD again.

6)Does it work now?

7)If it still doesn't work, it might be a bug in the ROM. See if the developer can help.

8) Of course, the culprit is probably a bad SD card. Try replacing it.