e0008105 - Invalid Physical Volume Library Media Identifier.

I am looking after an old 2008R2 server running BEW 2010R3 SP4 that is backing up a 2010 Exchange server to a series of USB drives. I am trying to recover some emails that someone deleted and I thought I was not doing a proper inventory or catalogue of the drive but when I tried to restore an email off of last nights backup I get the same "Invalid Physical Volume Library Media Identifier" even though the backup just finished and the drive has not been detached.

Any suggestions?

Peter