I got nowhere with all the suggestions below so raised a case with WD who quickly sent the following which immediately solved for the exact issue.For me this issue arose after upgradingto OS5 and with use of WIndows 10
Thank you for contacting Western Digital Customer Service and Support. My name is Bochra C.
Please try to do the 40 sec reset, also known as System Only Restore.
You can refer to the link below:
How to Reset a My Cloud (single bay) Device
https://support-en.wd.com/app/answers/detail/a_id/24022
Thank you for choosing WD Support.
Sincerely,
Bochra C
Western Digital Customer Service and Support
https://www.westerndigital.com/support