Beats me. The device has two partitions: data and backup. I noticed that when I checked the data partition, many of the directories seemed to be incomplete with files missing. Yet when I mounted it in another laptop everything was OK.
It is possible the error was caused not by the files being corrupt but by how the disk was mounted. When previously mounted in ‘untrusted’ the partition entry in the file manager was duplicated - with one entry greyed out.
Both the disk and the cable (usb-C to usb-C) were purchased new in the last month and were both used for the preevious backups.
I mounted the disk into ‘personal’, using a usb-A to C cable, and the restore worked like a charm (much to my relief).
Another slight difference was that my initial attempts were through the C-type port on my laptop whereas the successful restore used the usb-A port on a usb hub.
Thus I am not sure what caused the problem: the cable, port, original mount or some combination!