Emptying the trash after deleting Time Machine Backups
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.