Try the following as a troubleshooting step if you haven’t done so already.
Unsupported File System when reconnecting USB exn2
https://community.wd.com/t/unsupported-file-system-when-reconnecting-usb-exn2/238129/2
mephistopheles93
[Jun '19]If anyone is interested I was able to resolve the issue by connecting the USB drive to my laptop, running the Windows disk error check utility, clicking “safely eject usb device” in the systray and reconnecting it to my NAS. I think disconnecting it without safely ejecting caused lost/corrupted background files that windows ignores, but trips the NAS OS up.
And confirm one is using a supported file system on their external USB hard drive.
My Cloud: External USB Drive File Systems Supported
https://support-en.wd.com/app/answers/detail/a_id/19225
