I myself have switched to this single drive per OS myself after having problems with multiple OS's on multiple partitions, so unless its a laptop in which only 1 drive can be installed, I have multiple drives in use for this setup.
I have a triple booting system currently where I press F9 at boot and tell it which drive to boot from, or if I dont press F9 for boot preference, it boots Windows 7 64-bit. I have Linux Mint 18.1 as well as Windows 7 64-bit and Windows 10 64-bit. When one of the builds gets messed up all I need to do is detach the other drives and push image to that drive alone or build it fresh, then when done connect all drives again in my desktop tower.
If you want one OS to boot before any others as a default you can connect that drive to the lowest SATA port value, or lowest HDD SATA port value such as SATA 1 with Optical Drive at SATA 0.
My configuration is:
SATA 0 = DVD-RW Optical Drive
SATA 1 = Windows 7 64-bit = 240GB SSD
SATA 2 = Windows 10 64-bit = 500GB HDD
SATA 3 = Linux Mint 18.1 = 160GB HDD
Yours could be:
SATA 0 = DVD-RW Optical drive
SATA 1 = Windows 7 64-bit = 500GB HDD
SATA 2 = Windows 10 64-bit = ( Whatever drive capacity you decide to go with on a clean build of Windows 10 )
If you boot it and dont press F9 then it will boot to Windows 7, otherwise press F9 and select the other drive to boot from to boot Windows 10.
F9 Please select Boot Device
Reads all internal and external Drives and Thumb drives and allows one to pick which one to boot computer from.
*** If you have a laptop it will be kind of a mess to pull this off on a single hard drive. *** Especially if your using the free Windows 10 upgrade from the initial 7 build on the same drive which would require lots of work to pull off, and might be above and beyond what we can share here since its a gray area that ComputerHope might not want shared here since Microsoft really wants people to step forwards to 10 and not be able to step backwards to 7 under the same license. The gray area is that only 1 instance of Windows under that key will be active at a time which it would be, so its not like piracy where multiple systems use the same key at the same time or on multiple systems, but its a very very gray area and I am holding out on sharing how to do this because I dont want to get into trouble here and Microsoft really wants the key to switch permanently to Windows 10 use from Windows 7 use and not really allow for you to walk on both sides of the line between 7 and 10 at will under the same license.
On my build I have a 3-license package of Windows 7 and converted 1 of 3 licenses forward to 10 under the free upgrade that is pretty much expired now, with exceptions.
