When your PC doesn't recognize a 2nd drive
Creeds installed an old hard drive into a new machine as a second drive. The PC froze at the Windows logon. So Creeds unplugged and replugged the drive. Now the PC boots, but it doesn't see the newly-installed old drive.
When you installed the old drive, your BIOS probably decided that that was the first drive--the one that boots. So it attempted to boot your old version of Windows and ran into trouble. Remember that when you install Windows, it configures itself to your specific hardware. Boot one PC's hard drive inside another and you're bound to have problems. (For more on this, see How to move to a new PC.)
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So why did it try to boot from the drive the first time, then failed to even see the drive after unplugging and replugging?
Here's my guess: After you unplugged the old drive, you did something wrong when plugging it back in. A hard drive takes two connectors--SATA and power--and both have to be plugged in properly. If you fail to plug in the power connector properly, the drive will appear dead. It won't spin up, its light won't come on, and the computer won't see it. If the power is connected properly, but SATA is not, the drive will power up and spin, but it will not be visible to the computer.
So check the plugs again, and make sure everything is connected. Then boot not into Windows but into your BIOS settings. I can't tell you exactly how to do this because it varies with different models, but when you boot your PC, one of the first things you should see is a message that reads something like "Press key for Settings." Press that key immediately. You'll find yourself in a full-screen menu. Find the option to control the PC's boot order or boot priority. Set the machine's original drive to boot first.
One more note: If you plan to use the old drive for backup purposes, putting it inside the computer is a really bad idea. A burglary, power surge, or the Cryptolocker Trojan could simultaneously destroy the files on your main hard drive and on the backup. Better to put it in an enclosure and turn it into an external drive. See How to make good use of an old hard drive for details.
Read the original forum discussion.