you can try a “secure empty” of the trash…
to do this hold “command” and “control” keys when clicking on the trash. (it might ask for a password once you select this option)
if that fails. try repairing disk permissions by opening disk utility, selecting the external HDD then going to the “first aid” tab then clicking the "repair disk permissions button.
if neither of these work there are more drastic options available, but they require terminal (console) commands.
