Numbers - “The document could not be autosaved” to external USB

This problem crops up from time to time and it’s annoying, but I usually duplicate the file and delete the problem one. I have done this today, but this time, the copy is also coming up with the same problem. And the same with the next, and the next. It’s not a big file - less than half a KB!

The USB pen drive is barely touched with files - I have used just over 1 GB of a 32 GB drive.

I need to be able to save this file!

The problem seems to be the pen drive as I have tried to save a document from Pages onto it and it comes up with pretty much the same error.

It was working absolutely fine earlier today and then, suddenly, it wasn’t!

What’s the work around, if there is one?

Working on a MacBook Pro.