Not sure that this is what you're seeing, dpstjp but I've never been able to complete a backup. The Backup Wizards always stops with "Unknown Error". I can restart the Backup wizard and then it will stop at some other point until there are one or two albums left.
Everything (MD600, laptop with Control:Mac) is hardwired into the same router, with no wireless involved in the backup. Backup destination is a hard drive built into the router (Airport Time Capsule).
I suspect there is one album that's throwing a wrench into things. Running the Library Interrogator (hoping that this will uncover data inconsistencies or corruption) is on my list but that involves getting a copy of Windows, setting up a VM with Virtualbox or something similar. Too much of a yak shaving exercise at the moment.
Soundserge