Time Machine: Couldn't unmount disk

Hi all -

I'm having problems with Time Machine; I am hoping this is easy, so you can look smart, and make me look dum (which is fine if it solves the problem).

I got a backup disc, and I am getting the following error when I try to use Time Machine.

This is a new WD easystore backup (it has hardly been easy), which replaces a previous disk, which was not working. I assumed the old backup disk was the problem but now it appears it may be my computer. It is a 2014 iMac, running Sierra 10.12.6.

Any thoughts or areas I should investigate? Thanks