Not being able to access the usb drive on the router (while your other machines can) is the serious red flag. It indicates a network problem on the particular linux machine that has the problem. However, troubleshooting a network problem on single machine means considering so many possible settings that it can be like a needle in a haystack, emphasis on Haystack. I am out of simple suggestions and worse even simple methods for dealing with a potential haystack. I have one suggestion, that you can certainly reject, that can simplify isolating where your actual problem exists.
If the problem is solely in the “haystack” of settings on the problem machine: You could run a Live Linux (usb any version) on problem machine and if that live version can see the usb drive etc., you have at least isolated where the problem is.
Honestly, if the live version can see the usb and the rest of the network shares you’re looking at a reinstall on the problem machine. As an aside, my approach to reinstalling is to image my problem machine/drive, reinstall and then if I need something that was on the old version I use the image inspector utility to extract those files from the ‘bad’ image - sometimes I even image the new version and reinstall the old bad version if I think I need something from the bad version I can only get while it is running.
Again, I can not think of any simple actions at this point, so if you decide a different action, that is totally understandable.
Thanks for your help. I'll let you know what happens.