Error 0x80070570 during backup

I got the above error when trying to do a full backup of my disks to a
removable hard disk (about 1/2 full). [Running vista home 32 bit]. I'm
somewhat worried and confused by this.

Firstly, I have run scan disk/chkdsk on all the volumes being backed up and
on the target volume, and the target drive, and nothing is reported.

Secondly, I don't seem to be getting any errors on the files I do access.

Thirdly, the backup doesn't say what file it is having problems with - it
would be a help to know, as I might be able to get it off the previous full
backup.

The backup appears to have done something as there are 52 backup files and a
catalogue file, but I'm not sure how I could proceed from there.

Given the importance of a backup, it would be nice if
a) the backup ran to completion (OK, it might have done, but the error
message is telling me nothing about that)
b) It could tell me what file(s) it was having problems with.

Anyway, what can I do to proceed from here?