*Not* make copy bootable?

[backerupper3160]

Is there a way to make the SD copy *not* bootable? (I want to try it this way because the clone was accessing stuff like Dropbox when I would boot up from it, I didn't want it to copy over a file with an older version accidentally; I created a separate partition on the same drive with DiskWarrior for when I need to boot from it for diagnostic/repair reasons.)

I thought this used to be an option years ago, but I didn't see that as a choice on the most current version (for SL). Also it makes it a tiny bit faster starting up when there's only one "alternate" drive to the main one.