I found this out the hard way, and have written this quick post to help those who have installed their new Windows 10 copy as part of the giant MS update, and had problems.
The problem : You’ve installed Windows 10 on a new drive or partition as a way to test it out.
Now, after installing it , your Windows 7/8 installation seems gone, and you’re left with just windows 10 and no way to get back to it.
The solution : Fix your master boot records.
The quick fix here (For me, and it worked like a CHARM) was to put in my Windows 10 USB installation stick (The DVD will work just fine too), clicking ‘repair installation’ , navigating to command prompt, then typing in the following :
After this, I went back one screen and I now had the choice to pick between Windows 7 or Windows 10. Now I can transfer over my Windows 7 files to 10 including all the drivers I had originally got for W10 in utter safety. I spent no less than 3 hours trying to find out how to recover my W7 installation without any luck. Hope this helps someone!