I had this issue on 1.8 a few weeks ago, make sure your not plugged in. What I ended up doing was scan disk my card in my pc (found errors) then re partition my sdcard in the phone, then re flash my rom. I also updated to 2.2.1. After all this my problem went away. Not sure if you have to do it all.
Sent from my PC36100 using Tapatalk