After trying many of the solutions here (reset system, put USB drive on PC and ejected etc) what ended up working for me was rebooting the NAS without any USB drives connected - then connecting them after reboot - both now recognised - Yay!
After trying many of the solutions here (reset system, put USB drive on PC and ejected etc) what ended up working for me was rebooting the NAS without any USB drives connected - then connecting them after reboot - both now recognised - Yay!