can't delete file

I've been deleting files like this every week for a couple of years. They're backup files and large so I must delete one to make a new backup.

The file is on an external drive. When I try to delete it I get the message "Folder access denied" (it's not a folder, it's a file) "You'll need to provide administrator permission to delete this folder."

It has never required administrator privileges before and I am an administrator. It simply won't delete.

Windows 10 v1809

Often it asks for that but clicking ok deletes it failing that what are the permission on the file if it's a backup that can run as system

Thanks for the replies but I downloaded fileassassin and deleted it.