Cannot delete files from external hard drive

I recently got a new Mac and have been trying to delete the backups from my old Mac.

Whenever I try to empty trash after deleting them, it says that 'folders', 'Macintosh HD' etc are in use, across 10 different backups. These things are not in use, but I have already wiped and sold the old Mac so cannot go back and delete them using that laptop.

My hard drive is insisting it has 400GB of memory full with backups, which I can't get rid of.