There are threads on this in the Calibre forum, but I'll post here too.
If you have Calibre running on a Windows XP PC, Calibre will not recognize your HD. Kovid has released some new drivers to deal with this. (Look here:
https://www.mobileread.com/forums/sho...d.php?t=190982).
You must:
1. Update to Windows Media Player 11
2. Install today's Calibre update (and probably subsequent updates if Kovid continues to tweak code for XP)