How to delete over a million files without having to be there to select "Continue" when item in use

Got a new MacBook Pro. Have incremental backups on external attached hard drive for my old mid-2010 MacBook Pro (Yosemite 10.10.5). But have an another 1TB HD with unneeded backups (Backups.backup.db). Thought I'd do one nice full backup on that drive. But not enough room, so want to delete those old backups.

I connected this 1TB drive to the older, mid-2010 computer, and moved all folders to to Trash, and selected "Empty Trash". Files counted up and up, and up! This was in the evening, so I let it do its thing overnight. Today, over a million (1,000,000) files to delete. So, to empty the Trash, I have to sit at the computer because I am continuously getting messages such as "The operation can’t be completed because the item “CoreServices” is in use." Or "System" or "Macintosh HD."

I can hit "Continue" and item is deleted, but, I can't sit here all day. (Now am down to 806,000 to delete). I understand I could use Terminal to do this, but I'm afraid if I hit the "x" to stop the Trash emptying, the computer will hang.

Anyway to automate "Continue?"

What to do? I just should have reformatted that drive if I had known better. Or, can I kill this process somehow and then use Terminal to empty Trash? I am somewhat familiar with Terminal commands if someone gives me the command. Thank you!