http://www.f-secure.com/v-descs/epbr.shtml
Sounds like a virus executed at some point and fubared the disk. Probably deleted some of the metadata necessary for the disk to mount. You need a professional to recover it.
[EDIT] Actually I could be totally off base. It could also stand for Extended Partition Boot Record. I think that some nonstandard mojo was involved in formatting the drive so debian doesn't know how to mount it now. [/EDIT]