I’ve had this happen to me occasionally… (still) using avast 8 on a Win7 system. More often than not, I can fix the problem by running
CHKDSK /F
even if CHKDSK doesn’t find any issues. This may just be a “fluke” of my particular system. But I figure it shouldn’t hurt anything to try. If CHKDSK does find any disk issues, they should be repaired.