Windows 10: Windows 10 Install says I don't have enough space

The partition marked System is always the one booting the OS(es) and the only one which should be marked Active.

With multiple HD's you have the choice to leave all plugged in during install to create a Windows MultiBoot Menu, or you can unplug the other HD so that the SSD becomes independently bootable via the BIOS.

If BIOS boot is not to your liking then you can install EasyBCD to the System drive to add the other to a Windows Boot menu, but the advantage is that the drives still remain independently bootable since boot code has been written to each now.

Can we see another screenshot of Disk Mgmt to see how this went?

 

From his screen shot, Windows 7 is the E: Drive. You don't need to install EasyBCD to set up dual boot. From Windows 10, open the Admin command prompt then type: bcdboot E:\Windows. This should add an entry to the BCD for Windows 7.

 

Here is the current Disk Management.




I changed the boot priority to Win7 first, and now it is working fine. I can boot into either 7 or 10, and am slowly downloading tools and programs into 10 to test them.

 

You just moved the System boot files from the 100mb System Reserved partition intended to boot Win7 (strangely mislabeled "Data) to C. If you had Disk1 plugged in when you did this then the Repairs will have made its OS's also bootable from C so that Disk1 is not (or no longer, if it ever was) independently bootable.

Yet Disk1 has a System Reserved boot partition too, marked Active pointing to it as the intended boot partition for that drive. However when it is not booted we cannot see if it also marked System confirming beyond doubt it is the partition which boots the OS's on Disk1. I doubt it, since you had to set Disk0 first to boot in order to boot Disk1.

This configuration is problematic and needs to be studied from all views.

Please post back a screenshot of Disk Management from Windows 10 now. Right click the Start Button to select Disk Management.