I wanted to boot my home truenas box off usb flash, but I also know how flakey usb flash can be, so I use a pair of drives in a zfs mirror.

I would not recommend this setup, Any real drive would be better. I just hated wasting drive bays for the non-critical boot/system partition. In my defense despite burning through many cheap usb drives I have not lost the boot yet, however I do keep a new spare drive taped to the unit.