Unable to share from external hard drives

I have finally got my Xbox 360 working as an extender, and was able to view
my videos, etc. that I have stored on my external hard drive. I was sharing
them through Media Center, as well as through Media Player. Anyway, one
night I shut down to let everything cool down, and now my Xbox won't/can't
see the media that is on my external drive. If I check the library on media
center and on media player, the drive info is there. I have even tried
removing it and then adding it back, but that hasn't helped.

Any suggestions? I am using Vista Home Premium.

BTW, I can still see media files that are on my internal hard drive - just
not on the external drive.