I seem unable to change the external drive that file history is meant to use. I changed the drive volume name and size slightly, however I get the following errors:
1. It keeps showing the previous external drive details, with "unknown error" below it
2. When I try to select another drive, I get an error message box stating "we can't copy files to this location. your current File History drive is disconnected. Reconnect the drive and try again". If I press "run now", it again asks that I reconnect the drive, and will copy files to a hard drive...

I changed my mind about what I was doing with File History and changed something about the drive. When I did that I got a message...something about detecting a previous version of file History and did I want to use it Yes/No. I kept on clicking "Yes" thinking it was detecting the started backup on the drive and thinking it was asking me if I wanted to continue.
When in fact what it was saying was it knew I had previously started and it wanted the old drive. So every time I connected and clicked "Yes", it wanted the "old drive" and all it could see was a drive that was new to it. When I finally on the 5th try clicked "No" it worked.
“we can't copy files to this location. Your current File History drive is disconnected. Reconnect the drive and try again”
I think I read that as meaning "we can't copy files to this location [because there's something wrong with the drive you're pointing to]" when actually it was saying it can't find your old drive and you asked us to get files from it.