Before setting up Mr. Tb's new desktop PC (Win7), I copied all the documents from his old PC (I thought), via my laptop (because he only had USB1 ports) to the external hard drive I use to backup our home-based computers.

A couple of days ago, he went to update an Excel file and discovered that the folder that should have contained it was empty.

We plugged the external HD into his new PC and discovered the that folder showed as empty for that and 2 previous backups.

Fortunately, we still have the old PC (he's using it to edit music using some very old software), so we were able to copy the folder to a CD and use that to copy the Excel files to the new PC.

He was surprised to discover that the folder also contained some JPG files- apparently scans of pages from books (I'm not sure if he scanned them or downloaded the scans).

Does this make any sense? Should I be worried about using this HD to backup my laptop?

I would try to find out why it skipped that folder, which is probably the software. It may have skipped any folders with images, temp files, open files, etc. That directory could somehow be set to be excluded, etc.

I wouldn't worry about the drive yet.