At some point, you started iTunes without your external hard drive turned on. At which point, iTunes lost track of everything.
I'll be back, off to search for the solution, which I've seen recently...
(I trust that was the Jonathan Coulton cover of Baby Got Back?)
---------- Post added at 02:27 PM ---------- Previous post was at 02:25 PM ----------
OK, there's a few options listed under
macosxhints.com - One solution for iTunes 'can't find file' problems, be sure to read through the comments before proceeding.