"NTLDR is missing" error on Windows 7

I was running a dual-boot XP Pro/Windows 7 box, with Windows 7 running on a second SATA drive. Finally decided to move the drive inside and surplus the XP drive. I hooked up the new drive internally and booted up, only to see it throw this error:

NTLDR is missing.

Press Ctrl+Alt+Del to restart.

No amount of rebooting helped. We tried using the Winternals boot disk to no avail. Finally did some research and found a forum thread that helped.

Two issues were going on: I had another external hard drive that was set to "Active" in Disk Management. The second issue had to do with the missing XP drive; apparently, the dual-boot menu was stored on that system. Following the instructions on the Microsoft article, I ran the Bootrec.exe utility with each of the switches, each time testing using the /ScanOs switch. It kept showing that there were 0 Windows installations.

But I crossed my fingers and rebooted anyway. Lo and behold -- it worked! One or more of the switches fixed it. I booted into Windows, logged in, and Windows automatically downloaded and installed the latest drivers for my system -- including dual displays.