Is this a thumb drive or a larger USB drive?
It looks like it was originally formatted on a Windows computer. Do you know if it was exFAT? Have you backed up to this drive already (yesterday?).
I suppose you have already tried to eject the drive and remount it on the Nuc?
You might want to take a look at this post and try some of the steps suggested there.
If no luck there, @support will need to take a look at what’s going on.