Folder cannot be deleted from Trash
I have two suggestions. My first is to try creating a folder called .Trashes elsewhere and moving it such that it would overwrite the original folder; it's possible that this would leapfrog the problem (I used a similar method to fix a problem I was having a few months ago).

Alternatively, if the external drive is FAT32, have you tried booting a Linux distro from a USB drive and using that? Alternatively you could plug the external drive into a Windows machine (Boot Camp/VirtualBox?) and see whether that can delete the offending folder.