Windows 10 with 4 users 2 admin, 1 MS, one standard user (call him user STD). Drive Z: is a partition of D: ( C: is SSD) and drive Z: is shown in the File History "select drive" list for all users except user STD. Drive Z: is also shared as 'backup'. Tried many things (including reformat of z without success. Note::even though Z: was shared as 'backup' the share name did not appear in 'Network' and I could not find it by searching for it. Had planned to use File History 'add network location'.

Solution was to utilize the 'add network location' and enter the share name (from Z: properties //XPS/backup ) into the 'select folder' field (did not appear in available selections). I then started File History and had it copy its files. I then checked that a directory Z:\FileHistory\STD was created and that files were added to it.
I then stopped File History and again did a 'select drive'. Both drive Z: and the share name were now both available in the list. I then selected drive Z: and turned File History back on ( was prompted to select the newly created STD folder) and everything was now working properly
I spent a lot of time looking for someone with a similar problem without success. Then spent a lot of time finding a solution. Hope this helps someone out or leads to discovery of the underlying problem.