External HD not backing up Macbook Pro

I have a Trekstor Detestation maxi light 3.0 external drive that I use to backup my Macbook Pro (2019)

I’ve been using it now since 2014.

It no longer shows up in the sidebar, compared to another backup drive that does.

See screenshot of Disk Utility. The disk is clearly full. Recently the Trekstor had been trying to create free space, which went on forever to the point that I had to interrupt it. Then it got on  with the usual backup.

Problem now: no longer appears in sidebar and doesn’t backup.

Is there anything that can be done?

Erase content from Disk Utility, for example?

Restore?

Many thanks. 

Joseph Quinn