New
#11
You'd have to SysPrep the HD from the other machine to expect Win7 to start up on the new machine.
It may have detected the problem HD because you cabled it correctly when adding the second HD. Were these master/slave on IDE ribbon?
Finally, when you installed to target HD it configured a dual boot with Win7 using the System boot files on existing Win7 so that it wouldn't start when you removed the that HD until you ran Startup Repair to write the System Boot files to the target HD.
If you'll post up a screenshot of your maximized Disk Mgmt drive map and listings, we can look it over for you closer. Use Snipping Tool in Start Menu.