After much work, I've tracked down the most probable cause of my backups failing/hanging as a DCOM error with the Removable Storage database. I get event 10010 (The server {D61A27C6-8F53-11D0-BFA0-00A024151983} did not register with DCOM within the required timeout.) This GUID refers to the Removable Storage service. After following the instructions in Q235032 to repair the database, the problem still hasn't gone away.
I'm lost!!!
I'm lost!!!