It's not Seagate's fault.

Originally, I suspected the fixed 5-minute idle shutdown of the otherwise marvelous device would be causing the backup to break and generate annoying application popups and event log entries.

But it ain't so.

Apparently, thou shall not compress large files on fragmented disks. This in particular true for full backups to external hard drives.

Thanks to "jim".

In my case, the full backups consume roughly 25GB. I do not have problems with the incremental backups because they (no surprise) are usually much smaller.

So the solution should be defragment, then backup. If failed, rinse, disable compression and backup again.

Now, three questions remain:

1. Why don't they fix this?
2. Why, o, why, don't they have compression in the backup in the first place?  - To help sell tape backup devices?
3. And, shouldn't I be using a "real" backup solution?

Backup programs seem to be all crap one way or the other, so I'm rather not spending money.