I think I understand what you want to do. One concern I'd have has to do with the HDD you're going to put the Windows 10 upgrade on. As you probably know, your computer has to have a valid copy of Windows 7, 8 or 8.1 on it first, along with a valid product key. That product key came with the computer when you purchased it and it was sent to Microsoft when you registered the OS that was on the computer when you bought it. That registration also told Microsoft some other things about your computer such as RAM, processor, video card, size and type of HDD, etc. This data about your computer helps Microsoft to validate that when you upgrade your computer, it's the very same computer that you originally registered with them. If anything changes, you'd have to call Microsoft and explain the change and then hope that they'd let you upgrade. Now, even though you'd be removing the HDD from your computer, I'd have to wonder whether or not you'd run into some validation issues by putting a different HDD in your computer while you waited to decide if you wanted Windows 10. It might not happen, but it sounds to me like you could be creating some problems for yourself.
I'd suggest this instead. If your computer has already received the little white Windows icon in your task bar that shows that they've received your online request to reserve a free copy of Win 10, do this instead. Here is a URL link to Microsoft: https://www.microsoft.com/en-us/software-download/windows10 This link will take you to a page where you can download the Media Creation Tool from Microsoft. With it you can download either the 32bit or 64bit version of Windows 10, and can either install it right now, or save it to your computer as an ISO file, and then burn it to a DVD or a Flash Drive. I used Media Creation Tool to install Windows on four of my computers and it worked perfectly. It's the same installation you'd get if you clicked on Windows Update, when you get the notice from Microsoft to upgrade. I also used the tool and burned a copy to DVD and another to a 4GB flash drive. Take a look at URL link. If you're computer has been cleared by Microsoft as okay to receive the upgrade, this is the way to go. When I made the DVD and Flash Drive, I simply downloaded the Upgrade to my computer's Download folder, making a new folder there called Windows 10, into which everything went. So now I have an ISO of the Upgrade as well, which I'll hold onto for awhile on an external drive. Hope this helps. This is a far less complicated way to get what you want and you won't have to mess around with your present hardware. Even if you don't burn the ISO to a disk right now, you can always do it later. Good luck!! :-)
Yes it should work if activated before the time limit of one year runs out. – Moab – 2015-08-02T01:39:33.743