Perhaps I didn't express myself clearly - I am doing a file backup of the whole C: disk.

But I have gone back and had another look at this problem.

In fact I have found that the Disk Master file explorer shows a folder called "SkyDrive" in the file list (which is the old name of "OneDrive")

Interestingly, doing 'cd OneDrive' at the MS DOS prompt shows no OneDrive folder but 'cd SkyDrive' works fine.

So Microsoft are doing some kind of fiddle here to make the "SkyDrive" folder appear as "OneDrive" in Windows Explorer.

Restoring a file from the DiskMaster backup of "SkyDrive" makes the file appear in "OneDrive".

So, problem is solved - I can live with that.