While you might use a BIOS boot selection to chose which HDD to boot from, usually we install the newer OS and make it dual boot at that time. It looks like you didn't do that so your only exit is to see if your PC allows you to select which drive to boot from.
Here is the question... I have a computer with vista and a hard drive with windows 10. Is it possible to put the windows 10 drive in and have the computer boot from either vista or 10??

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