I'm not super familiar with the WD mycloud but I would hazard a guess that they use a proprietary file system that isn't recognized by the new NAS. If that is the case you'll likely need to reformat the drives to make them work with the new NAS. If you have a PC available you may be able to copy your old data before formatting the drives but that can get tricky with having enough storage available.

If it were me I would buy new drives for the new NAS, use a PC to copy your old data to the new NAS then sell or repurposed the old drives