Thanks nukecad, I appreciate your help. Both external 10TB drives I have here were working perfectly until I installed a PCIE NVME+RGB add-on card a few days ago. I'd not fired up Emby until just yesterday and only noticed problems with the drives at that point - one drive (WD Elements) was outright invisible to Windows 10, the other (Seagate Barracuda) was recognised in Disk Management but needed to be initialised/reformatted.
I did notice on the WD drive that the tape used to cover over the first three pins had worked its way loose (common WD hack). So perhaps it was that? I am considering perhaps investing in a replacement 10TB drive, but it's a heck of an outlay $£. However, one thing that may have happened is that I could have inadvertently put the same disk label on the PCIE NVME 256GB drive as was already in use on the WD drive, maybe that caused a conflict when I fired up the external 10TBs?
Anyway, thanks again for your help!