Here's what you're running up against: Most modern drives do not allow you to format at anything other than 1:1 interleave. So, HD SC Setup can make all of the requests it wants, but the drive's own controller will blithely ignore all requests to format at other than 1:1 interleave.

I'm actually amazed that you were able to get the Plus to boot off of a 4GB drive! That is a *very* rare achievement.