I had one last failed attempt via USB (reformatted again on Windows, no joy seeing it on the Innuos Zen Mini Core, then formatting again on the Innuos, before backing up (successfully) from the PC (direct to the USB). Then no joy again on the Innuos restore, even after rebooting it still couldn’t detect the USB drive.
However, my latest, and final attempt, at restoring a backup was back over the ethernet mid morning. (I would never find time for all this at any other time of the year). It worked…finally 
I have come to the conclusion (only a theory) that any backups I made to folders on the PC that sync with Microsoft OneDrive, do not restore to the Innuos. At one point, I saw a strange error message when copying one such backup to a USB, saying something like “[certain] metadata information [which looked like the sort of thing OneDrive uses to monitor the syncing status of some of the files within the Roon backups] couldn’t be copied”. It errored on files, that for some unknown reason, would not sync to OneDrive cloud servers.
The backups direct to USB failed for different reasons, but a backup to the root user folder (not synced with OneDrive) restored successfully over the network to the Innuos no problem at all. I am yet to see how the Innuos now performs as a core, but the restore was a success.
The solution for me was not to try and restore a backup to a folder synced with Microsoft OneDrive and to make sure SMB1 (whatever that means) is installed and activated on Windows 10, also see this post…