Inhibit backup on special directories

[Ismael Valladolid Torres <ivalladt@punkass.com>]

My USB disk mounts on /media/usbdisk. I'd like that, when editing files
located on that unit, backups are never created, at least on that
location.

I've come to this snippet:

 (setq backup-directory-alist
    '(("^/media/usbdisk/" . "/tmp")))

This works, it moves backups of all files there to /tmp, but I wonder if
I could simply inhibit backup file creation at all.

I see there's a variable called backup-inhibited, but I don't see how to
make it work for files in a given directory. Well, sorry I am not an
elisp expert by now!

Any ideas useful.

Cordially, Ismael