Vista Backup Error 0x80070002

Running Vista Home Premium SP1 on a Toshiba notebook with 2 hard drives...
I've been running regular back ups C: to D: but it's started to fail with
the following error:

'The backup did not complete successfully.
An error occurred. The following information might help you resolve the
error: The system cannot find the file specified. (0x80070002)

I tried starting a new "full back up" and it failed with the same message...
I don't want to delete the old back up, at least not yet... does anyone know
the fix ?

Thanks... m