I wonder if someone can tell me what's going on here. I installed Windows 7 Pro (64 bit Upgrade edition) tonight. I have two hard drives in this system, one of which is old and has a Windows XP install on it. I was planning on retiring that drive, but I'd heard that if I kept in the computer while installing Win 7, the installer would recognize it as a valid upgrade platform and let me smoothly install onto the other hard drive (a newer one). Well, that much seemed to work. I installed the program and entered my serial number and everything is working fine... except that when I went back into the computer and removed the old hard drive, suddenly when I tried to reboot it was complaining that there was no system disk installed. I wondered if I'd somehow accidentally installed it on the wrong drive (even though I was sure I hadn't), so I tried taking out the new drive and putting the old one in alone. This time it somehow recognized a Windows 7 installation (giving me the option to boot between "An older version of Windows" and Windows 7) but when I tried to boot it complained that a device is missing. I put both drives back in and everything works again. So it seems that this installation requires both hard drives to be in the machine, but I don't understand why. Is there something I can do to get rid of the old drive for good?