Booting the raspberry pi 2 from a usb hd

[Victor]

I'm using a pi 2 card with FreeBSD 11.2-STABLE on the required sd card. It works like a charm but using it as a postgresql server with many write operation on it, to be on the safe side, I would like to move the operating system to an external hard-disk or ssd. 
I've been surfing the net to find if it is possible to boot my pi 2 from an external hard-disk (or SSD) to no avail.
Could you please give me some hint, directions on this subject?
Ciao
Vittorio