CleanmyMac X emptying trash

I'm using an early 2015 Macbook Air 13 inch, so 128GB. 250GB Seagate HD as backup. Time Machine backup was failing, so I moved most backup folders to Trash. Tried to empty trash, but kept on hanging up saying something was in use (Library, usually). So I tried Trash Bins on CleanmyMac X. 24 hours later it's still running "Calculating size of trash bins" and is up to 4.154TB ! I can only assume that each backup folder is incremental from the last, but this is calculating the complete backup x the number of folders. The blue circle surrounding "Stop" is tantalisingly close to complete, but was this morning! Guess I'll just have to let it run and see what happens. If I stop it I'll still have an un-empty-able trash bin.

Fortunately I have a second backup HD stored in the shed, and swap them weekly.

Any thoughts or advice, please?