BB your situation was a bit different. You started with 2 installs of Windows 7 and upgraded one to Windows 10. This automatically took care of the boot loader to handle both.
In John's case he is starting with Windows 10 and is thinking about adding Windows 7. If he installs Windows 7 on the second drive the option to boot to Windows 10 will be gone and the system will boot straight to Windows 7. This is because Windows 7 'knows' nothing about Windows 10 so cannot detect to set up the dual boot menu.
Of course Windows 10 can be added to the boot menu easily with EasyBCD but it may be difficult to give instructions on the forums; of course screen shots could be supplied to act as a step by step guide but it would be difficult for me to supply the screen shots as I am not in a situation where I could go through the process to capture the screen shots.
Hmmm, I suppose that, if John decides that he wants to do this, I could use EasyBCD to remove the Windows 10 option on my laptop and then go through the process of adding it back. That is actually workable and I'd be willing to do so.
Another option would be to install Windows 7 to the new drive and then use which ever F# key on his system that would give the boot menu and he could chose which drive to boot from. If this option were to be used I'd want the SSD holding Windows 10 to be disconnected during the install of Windows 7 on the new drive. This is likely the better option in this case.
Still, before anything is persued in this direction we need to get him successfully booting to Windows 10 per thread:
viewtopic.php?f=25&t=1383