GPT Win 8 Dual boot installation killed Win 7 MBR boot option.

There should only be one EasyBCD listing for each OS. In Edit Boot menu tab, Delete all of them now except the one for the booted OS. Then add the other OS, reboot to see what happens.

I don't know why you are moving the Active flag without instructions to do so. It requires doing Startup Repair - Run 3 Separate Times to move the boot files to the Active partition on an MBR disk (not UEFI). You should have all other HD's unplugged to do this. Once Win7 starts on its own and is labeled System partition, you can then use the BIOS Boot Menu to choose it's HD to boot.

Enter BIOS setup to report back exactly what settings are available for Legacy BIOS or CSM. You need a Dual setting, which might also offer UEFI Boot first. You can then set your UEFI HD to boot first, trigger the other using the one-time BIOS Boot Menu key.

Back again. Issue resolved after reading over 100 pages of research online. It came down to restarting the computer in Win 8 from the Charms Bar, Settings, Change PC Settings, General, Advanced Startup, Restart Now, and eventually getting to the Command Prompt, and typing what I took a picture of in the Command Prompt.

I did switch the Active drive back to D: via command prompt earlier, and deleted the 2 Windows 7 entries in EasyBCD that were not working.

I did use EasyBCD to change the order & the default OS & the countdown timer to 15 seconds in the Windows Boot Manager, so that was very handy, thanks.

I'm currently making new backups of both drives before downloading the Windows 8.1 update.

The steps I gave you will resolve the issue. If you had problems getting into BIOS setup then you only needed to ask back. That's what we're here for. We've resolved this issue countless times before.