Quoth Russell Brown.....
Thanks for the replies but I'm trying to disable the device popups globally.
<facepalm>
I is a idjit :-(
I already had:
[Media][$i] enabled=false exclude=media/builtin-webbrowser,media/hdd_mounted,media/builtin-mycomputer,media/builtin-mynetworkplaces,media/floppy5_unmounted,media/cdrom_mounted,media/cdrom_unmounted,media/floppy_unmounted,media/hdd_unmounted
in my global kdesktoprc.
In the melee I'd forgotten that we'd changed to encrypted media for the backups as part of the great update project; so appending "media/hdd_unmounted_encrypted" to the end of that line solved my problem.
</facepalm>