I can't empty my trash of its 'empty' folders

[Nicolino]

I use an external drive via Time Machine for back ups from my iMac running MacOS High Sierra 10.13.2. It was getting full so just before Christmas I moved the 2016 folders to the Trash. All the Backup files have been removed but all folders labelled Macintosh HD, Applications, System, Library and Utilities simply WON'T go. I've tried all sorts of options like Safe Booting Mac, Holding OPTION key while emptying trash, using the right-click method to Delete Immediately...nothing! Once I disconnect the ext. HD from my iMac the trash folder is immediately empty. Reconnect and they're there again.

 

[Trees]

Is there a setting in Time Machine to let it free up space as needed? Maybe if that is turned on, then Time Machine will be able to clean up the empty folders.