Changed Time Machine hard drives
Use Disk Utility to reformat the old drive. It will be much quicker than forcing the Finder to delete hundreds of thousands, if not millions, of files by throwing it in the trash.

If you have other files on this old disk you want to keep on this drive, you can:


copy them to another disk; reformat the old disk; copy them back to old disk


or 


use Terminal to delete Backups.backupdb using the rm -d -r command. Using Terminal removes a lot of overhead imposed by a Finder delete and works much faster.