I am running 10.12.6 and have 5 old hard drives connected to my Mac, each of them had previous OS on them at one point or another.. I am trying to keep all the user/application and other data on them, but remove the OS system and library folders. The problem is, as soon as they get put in the trash, a new one is created on each drive that has many empty subfolders. I don't want any of these drives to show up as potential startup disks anymore. How do I clean these drives without reformatting? I was able to mount all the hidden EFI partitions on them however, and successfully removed all the EFI folders.