External Monitor on Windows 10 Boot Camp not detected?

I have an iMac 27 inch w/Retina. I installed Windows 10 via Boot Camp.

I added an external monitor (Toshiba TV Monitor which works fine as a secondary monitor with Mac OS X on this iMac 27 w/Retina).

The external monitor is simply NOT recognized in Windows 10. I rebooted with it plugged in, etc. I attempted to install a new Windows driver for the AMD graphics card, etc.

The weird thing is, I had a 21.5 inch iMac for a week (I took it back and upgraded to the 27 inch). During that week, I had windows 10 installed on it and the same external monitor worked fine, in both Mac OS X Yosemite and Windows 10. For real.

My conclusion is, one of two things is going on:

1) The 27 inch iMac w/Retina's AMD graphics card is the issue (as it is the only difference in the scenarios), or

2) One must have the external monitor hooked up and on while preparing and installing the Windows 10 installation. In other words, maybe Thunderbolt itself is not recognized because nothing was hooked to it while running Boot Camp Assistant. But I be ****** if I'm going to reinstall Windows 10 again.

Any idea? Any way to tell under Windows 10 if Thunderbolt is working? I have no other Thunderbolt peripherals to try.