I went to my Time Machine External HD and deleted one of the backups by dragging it to the trash. Now when I try to empty the trash I keep getting messages that certain folders can't be deleted because they are in use. How do I empty the trash completely? I tried opening the trash and right-clicking on the folder and choosing "Delete Immediately" but it still gives me the message that folder can't be deleted. I'm running High Sierra 10.13.5 on an iMac.
Of course the problem only shows when I want to empty my trash and my related Time Machine volume is mounted. When I want to delete my trash, I temporarily unmount the TM Volume. And from now on I will not try to make extra room on my other TM back-up volumes by deleting folders from my Time Machine back-up. I will either let Time Machine automatically delete what is needed when the volume is full, or manually delete data using a method built into the Time Machine software.