I think that there’s a couple of issues here.
First, your local music files are held on the WD USB drive. I suspect that this has been formatted as NTFS, rather than the recommended ExFAT.
When you removed it from the NUC (which is running a version of Windows, I think?), it may not have been removed safely, and now the Nucleus+ won’t be able to read it when it was reattached to the Nucleus+. See this post on what you need to do:
The second issue is that you have done a Settings and Database reset on your Nucleus - so you have wiped out the Watched Folder setting for the USB drive. You will need to set this up again (in Roon’s Settings > Storage screen).
Be aware that only internal 2.5" drives show up in the status screen of the Web Administration Interface page - USB drives do not show up here.