Hi,
Many thanks to Richie66 for his post.
Western Digital drives can be a pain in the neck - my two externals just vanished and became 'Unknown Devices' to Vista.
After searching here I put them on their sides, replugged them and hey presto - there they were! A bit of switch cleaner in the sockets and they now seem fine again.
I had a My Book World which was such a pain I ended up taking the hard drive out and installing it internally.
Time WD got their act together!
Cheers,
Barryt