Surface 3 128GB with 128GB microSD card
External empty 3TB WD hard drive formatted as NTFS

Configured backup to backup both the internal SSD and the microSD card, total used about 100GB.

Backup repeatedly fails with:
Check your backup
The last backup did not complete successfully

Clicking the More Information button yields a box stating:
Check your backup
Windows Backup encountered an error when writing data to the
backup target.
Details: The system cannot find the file specified

There are two options:
Try to run backup again (this I've done at least 10 times with the same result)
Change backup settings (not sure what I'd change)

BTW, the backup seems to work on my HP workstation but on that machine I only backup the C drive. Everything else is backed up with Genie Timeline.

Any thoughts as to the cause of this error? Web searches don't provide any help. One hint: the amount of space consumed on the external drive approximately matches the used space on the internal SSD. Win10 on Surface tablets has multiple problems with the microSD card slot, could this just be another of them?