Try sd card to other phone. If it works, do this:
Save sd card content to PC, format sd card, go to recovery, wipe data or partition sd card to your needs, reflash ROM and GAPPS. If still unmounts could be a bad sd card.
it only does this with this rom, and it does it everytime i reflash this rom with clean wipe and stuff like that...
any other cm7 or Gingerbread based roms work...