Thanks. I tried a Pre-clear with the plugin but unraid needed the unassigned and it all started to get a bit complicated with them being assigned to the array.
In the end I pulled the usb, used parted magic erase disk which I use a lot, so I know how it works. https://partedmagic.com/secure-erase/
It's probably just a fancy ui fror hdparm UhClem because it erases at the hardware level so no load on the system at all, which is sweet. Takes about 240-338mins per drive and it can clear all 5 at the same time. I'll test a drive with a recovery program afterwards, and if all clear I'll sell it.
Not worth that much with drives with 3 years running time, but better than cluttering up my cupboard.
Thanks for the help everyone.