iPod not able to find some songs???

[skybolt]

Recently, when I update my iPod (5.5G, 80gig) I am almost always told that song XXX could not be copied to my iPod because it cannot be found. Sure enough, in iTunes, there is the dreaded exclamation point! If I say to locate, it cannot find it anywhere. My question is, where did it go? I have not moved anything, and except for these few songs (and podcasts, I migh add), everything is fine. I do keep my library on an external HD, but that should not be the problem, all else copies just fine.

Any ideas as to where the songs went? And how to find them again?

Thanks in advance!