**I'm coming from Windows 7 Pro management, adjust my ideas to suit your Windows OS **
If you desire to re-order your HDs eventually, first consider do this: give each hard-drive partition a unique name. For example, I have my internal hard-drive's C (OS) partition and my D(ata) partition named S03[174]C and S03[174]D respectively. My other laptop uses YT0 rather than 174. One laptop is S02, the other laptop is S03. Reason for 174 and YT0 -- the hard-drives contain those numbers as part of their respective serial numbers. What that does: no matter what USB or DVD is booted, no matter what internal OS is booted -- I instantly know each partition and each hard-drive via any disk manager, backup/restore utility, and so on. Drive letter assignments by such boots don't matter, I still know each partition and each hard-drive.
Now, how does all of that apply to you? If or when you decide to physically place the hard-drives on the cabling / in the drive bays -- in the order you want them to be; upon any Disk Management utility USB or DVD boot, you will know which partition should be Active (System Reserved partition for Windows 7), you will know to mark C (OS) partition Primary (and Active if no System Reserved partition). I think all other partitions that you created are marked Logical.
There are some partitions created by Windows 10, Windows 8, Windows 7 -- these should not be label-changed nor drive letter assigned. For example: recovery partitions created by computer manufacturer, boot partitions by Windows.