Failed with an unknown error.
When I booted-up this morning I got a dialog which presented the above message and no other information!
The dialog is "helpful" in that it tells me something is wrong, but I have no idea what it is referring to; so it isn't actually helpful.
What might be causing this "backup" error when I am starting the system?
I've restarted several times and keep getting the same message.
I checked the disks using System--Administration--Disk Utility via a Live-CD, and it reported no errors.
I've just now noticed one tell-tale sign which is probably significant, but I don't know how to interpret it...
My normal routine, with two of my drives (single partition per drive) is to manually mount them (via Nautilus)... However, what I've just noticed is that one of them is showing as already mounted at startup, which has not been the case prior to today...
I haven't got the hang of this mounting "stuff" yet.... but this seems to be a relevant point...