External hard drive and iTunes

I bought a 250GB freecom external hard drive yesterday and the reason I bought it was because I didnt have enough room for all my media on my PC.

Ive cut all the media from my PC to the external hard drive and sync-ed it through iTunes and everything was working fine through there to sync it upto my iPod.

I turned on my PC today and iTunes is coming up saying that the file (music and videos) could not be found, although the external drive is turned on and I can access the media through windows explorer and play them (so its not a corruption issue).

Does anyone know why this happens and how to ammend it?

Thanks, Matt.