Comment # 8 on bug 954650 from Stakanov Schufter

This seams to be at the end a hardware limitation of the bios of the machine.
If one installs with one disk at the time it is possible to have a machine with
dual boot via Bios. 
So on very old machines with a agp/pci-e combination for graphics and a
risercard for a cpu upgrade etc.etc. you may encounter this. 
Solution: In this particular configs one has to use a screw driver. Once
installed, connecting the disks, dual even triple etc. boot is possible via
bios selection of the hdd to boot.