I don't know what it is but my WesternDigital 320GB external hdd's have buggered up every now and then, I've lost write access, partitions have dissapeerd not to mention the lousy cables they came with. Just to use them I have to unplug my LG Dvd drive and use it's usb cable to use my hard disks.
A reformat fixes the strange permissions problems but it is a pain to copy from one drive to the next in four gig batches at a time from one drive to the next every six months.
I realy only use them for music and pictures, but there should be a better way to deal with this.