If you have a valid install on sda and you want to 
put another install on sdb you cannot as it assumes you
already have a root drive on sda and doesn't give you 
the option to put one on sdb.

Example- we have a few machines that have multiple
os's for testing and we select using the bios as to which disk to boot
from.  sda have all sorts of stuff on it. sdb is the users 'working'
os. user cannot install to sdb as the installer thinks there is
already a / on sda