I know that I saw this recently.
However, I can't find it again to see if anyone has found a fix.
Since upgrading to WM5, I have occasion to lose my CF card.
I have to do a hard reset to get it back . . . . and then it goes away again after a few hours.
I also use a 1 gig SD card with no issues. Just the 2 gig CF card seems to go away.
Anyway, if anyone has seen a fix for this, I'd appreciate a call out.