ERR: Used disks. Check RAIDar (dirty disks)

So after years of faithful service my RN 314 has this error message on the screen.

RAIDar is telling me to reformat.  I would rather not, as I've just had my backup USB disk that I use for backing up the NAS die yesterday so I don't have a backup.

I've searched the forums and there seems to be a solution available but its not shared?

Firmware is v6.6.1.  I changed nothing on the NAS recently (haven't touched it for some years, it just soliders away).  Just now its throwing this error.

Any help would be greatly appreciated.

I had to register a new username - I used to be very active on these forums.