[Trisquel-users] How to assign a backup drive during the install?

[mcz]

Assuming I can boot from a LiveCD soon, I'll have to figure out how to  
install according to 3 drives: 1 (main) ssd, and 2 hdds.

I'd like / to be on the ssd, /home on the first hdd, and the second hdd is  
for backing up the first one.
I don't need to backup the ssd as long as I get a copy of the config files I  
use.

Setting up / and /home, I know how.
But what about the last one?
I assume all need to be formatted.
Most likely all to ext4 (not sure about btrfs yet).