Oh no, Windows 10 tomorrow but I'm switching hard drive, what do I do?

Sorry couldn't write a proper question in such few words. I'm all set up for the switch to Windows 10 from Win 7 but a thought just occurred to me (not a great time either seeing it's last minute)
Since i'm leaving the country I'm going to remove my flash drive where the OS sits now, and replace it with the old drive that used to be there. This PC will remain here as is so what do I do?
Shoot, question is still blank--sorry let me explain some more. I assume that windows 10 will install replacing the present one if this drive stays here. BUT since I will removing the drive and replacing it with the previous one--same OS license since it wasn't isn't a new PC then the old one has the following things:
-1- about a year's updates to do
-2- no 'Get windows 10" icon sitting on the bottom left of my task bar.

So obviously I should NOT install windows 10 on this present drive since it will be wiped clean once I remove it. But how do I tell Microsoft this? "Dear microsoft btw it's not this drive that wants win 10 it's another one?
and
Since by the time I switch drives Inn 10 will be out do I still need to do all the win 7 updates and then switch to 10 or what?

Sorry for all this confusion but hopefully you see where I'm coming from with this problem.

Thank you