Assuming you are using a 80-conductor IDE cable, whatever drive you put on the end connector (the one farthest from the motherboard) will become the master and will be the only drive you can boot to. There is no easy fix for this. I'm not familiar with Macs but am about to build a hackintosh in a couple of months. If your Mac is capable of using SATA drives, I would clone the drives you are using to new SATA drives. Use a program like Ghost or Acronis to clone the drives. Then you can build a switch to switch between the SATA drives. (see thesataswitch.com for directions on how to build such a switch. Hey, if you can afford a Mac, you can probably affor a couple of $70 hard drives!