deleting hard drive backup

Hi, I've a separate hard drive that backs up my desktop imac. When I first bought it I also used it to back up my Macbook, but soon switched to using separate hard drives for each computer. However I've still an old backup of the Macbook on the first HD which I tried deleting by dragging to the trash. Bad idea! The trash refuses to delete the old back up, I get "Emptying the Trash cannot be completed because "boot.efi" is locked. To empty everything in the Trash, including locked items, press the Option key while selecting Empty Trash". Needless to say, I've tried this, and it still won't delete the old files.

Can you help?

What are you doing the backup with?

Suggest going into Disk Utility and select the external drive with it connected and powered up, highlight it and hit Erase and then Format Mac OS Extended. Using your backup program immediately backup.