I have three hard drives and my intention was to create a dual boot system with Win 7 32-bit and Win 7 64-bit. Some of these hard drives (I forget which) had Windows XP installed on them in the past, but I intended to use them as data drives in the new system. I installed Win 7 32 bit on hard drive #1. It worked fine, although it was curious to me that when it booted it gave me a choice of selecting "Windows 7" or "previous versions of Windows." I guess it noticed Windows XP was on one of the drives.
My BIOS lets me choose which hard drive to boot from. So I thought, okay now I'll install Win 7 64-bit on hard drive #2 and in the future to choose which OS to boot, I'll just use the BIOS. So I install Win 7 64-bit on hard drive #2. It works fine-- when I boot from hard drive #2 it runs the 64-bit version.
Then I try to boot from hard drive #1. The BIOS says it's not a bootable drive. I'm thinking, "Huh? But it has Win 7 on it."
What I eventually realized was that when I installed Win 7 32-bit on drive #1, it didn't make drive #1 bootable. It actually put the boot partition/thingy on drive #2. When I installed Win 7 64-bit, I formatted drive #2-- then it again put a boot partition on drive #2, but now it has no idea that there's an OS on drive #1.
So what do I do to create a dual-boot system now? I don't want to have to reinstall Windows on drive #1 and I don't want to wipe my data -- but all my data is backed up, so I'm not worried about accidental erasure. I just want to try to get this working without reinstalling windows.