I tried moving an SSD with Win7 to an older computer, but this error occurs. The disk is detected in the BIOS, but no matter what I did, the OS refused to start. It still boots normally when I switch it back to its original HDD. What could be the problem?
The problem is that you cannot install a Windows system drive in to a different computer which has a totally different motherboard and expect Windows to start - it just will not work like that.
That would only work if the computer you are putting the drive into has exactly the same motherboard as the computer that the drive came from , and even then it's not guaranteed to work.
You must format the drive to erase the existing Windows installation, then install Windows from a Microsoft Windows 7 DVD.
Another consideration (though nothing to do with the above problem) is that if it's an old computer from pre-2008, the motherboard manufacturer (or PC manufacturer if it's factory-built) may not have any Windows 7 drivers for it, so I would check that first before going to the trouble of installing Windows 7 on it.