Just so you know, there is always one primary hard drive and one or more slave drives on a given computer. The primary HD has the master boot record referenced by the CMOS. If you switch drives like you're doing, it obviously doesn't work because the proper master boot record is not found.
You definitely need to use a boot manager program of some kind, like the one patch41 suggested. There are many choices out there, so I would do some research first before jumping into this, as each program has it's own reliability track record and capabilities.