I've had a similar problem with my external drive which is where I keep my iTunes library. I'm still not sure whether the drive or iTunes is to blame. I posted at the iTunes forum about it. I think you should too ... the more noise we make ... iPod Forum & check the index for iTunes on Mac. BTW sorry for the late reply but I posted on that forum because I wanted to get an answer also. But nobody was aware of exactly why. I think I received about 3 suggestions why this might have happened.

BTW you are talking about the iTunes Music Library aren't you? And not the LW playlist that appears in iTunes. If you're refering to the LW playlist in iTunes then the answer is because your songs in the LW playlist reflect your LW download folder (shared folder.) Since you changed the location of the downld folder & probably didn't move the files from the old location to the new one, then iTunes can't find them in the location the LW playlist would look for them. I don't use the LW playlist function in iTunes. I simply make sure all files are properly added to the iTunes music folder which I do manually. That way I don't need to keep my external drive connected the whole time I'm using LW.