After installing High Sierra can't mount external USB drive (TimeMachine), why not?
Ok something is broken in High Sierra disk utils (as of 2018/04/02). It seems unable to mount discs correctly. And I think crashes. I suspect it's something to do with the EFI partition but I'm only guessing.

To fix my problem I had to format the drive and start the backup from scratch. This did fix the problem.

However you can not format the disc from disk tools, it crashes. You have to format it via the terminal.

I followed this short guide.

https://mycyberuniverse.com/web/how-fix-mediakit-reports-not-enough-space-on-device.html

I then ran Time Machine and selected the new disc.

Not quite sure how a bug like this manages to fly under the radar at Apple. Poor show.