What are the drive requirements for File History?

I have been trying to set up File History for my father's Windows 10 Home PC. I've attached several HDs via a USB3 port but keep getting "no drives found" when trying to set up file history. Some drives were in a docking station and one (which I most want to use) is a 2TB Fantom (sp?) brand external USB drive. That one is USB2. The drives are always visible in Explorer and can be read and written to. I noticed that the File History service was not running so started it but still no joy.

The only thing I can think of is there is something about all those drives that make them unusable for File History OR something else not enabled on his PC but MS in its infinite wisdom either doesn't publish the requirements or hides them too well for me to find. Likewise web search didn't turn up any hints as to what is wrong.

I don't recall his Windows 10 version number but know it is up to date.

Any ideas?

How are the external drives formatted? (NTFS, FAT32, etc.)

Also, check for this -

Select Drive for File History in Windows 10

All are NTFS.

Had read that link before posting.

FWIW, the drives have other things on them, such as the folder where Macrium backups are saved. That shouldn't be an issue since I have Macrium backups and lots of other things on the USB external drive I use for File History on my personal Windows 10 Pro system.

I think that if the drives have any folders on them which Windows thinks it owns (eg re-located 'Documents' or 'Pictures') then FH won't use them (makes sense, as the backup would be in the same physical hardware as the source). So check exactly what folders are already on the drives. I wouldn't expect MR folders to be an issue.