Thanks for the information Abrahán González.
I have installed RaiDrive on my PC, and have not spent long on it, only using for the last half hour. Some comments:
(1) RaiDrive is free for personal / home OneDrive accounts.
(2) I have only set it up with my main OneDrive account (no other accounts or cloud services).
(3) It appeared to only give me the option of selecting a new drive letter ( I opted for drive letter O:) for the location to access the files, and not alongside my OneDrive folder on the D: drive.
(4) The files appear as if they are local. No OneDrive local (tick) or online (cloud) icons.
(5) So far, files open as if they are local, but surely cannot be, as I have ~1.5 TB of data, so I assume each file is opened from the cloud (I have a good internet connection speed).
(6) OneDrive remains connected as before, which will allow me to check it, whilst continuing to use RaiDrive, until MS fix OneDrive!
RaiDrive has been helpful in listing my files as if they are on the PC, like OneDrive was two days ago, but without the OneDrive extra features, such as version history, etc.
RaiDrive has also been very useful in allowing me to compare the shared folder with its duplicate folder as one would with any two local folders on a PC. It appears the duplicate folder was an incomplete duplicate, so I could make sure that there was nothing in the duplicate folder that was not already in the shared folder, before I deleted the duplicate folder.