I previously had a setup of one SSD partitioned in two (Yosemite on first, Windows 7 on second) and the original HDD as additional storage (two HFS+ partitions). I was going through yesterday to try to clean up and make the HDD have two partitions -- one FAT and one HTS, while retaining my backups on the HTS. Long story short, for some reason my Windows install would fail to boot after these changes (despite being on a different drive from the Windows and macOS installations).
I wiped the Windows partition and have been attempting to reinstall via Bootcamp. Unfortunately, my laptop monitor has been broken since I did the first install and I think that may be my problem. I load a bootable USB and partition through Bootcamp Assistant just fine, but upon reboot to do the Windows installation, I get a momentary grey screen followed by a blank screen (monitor turns off from no signal). It does sound as though the machine is working, so my hunch is that the MiniDisplayPort drivers have not loaded but that the Windows installation is present. If it makes a difference, the monitor is running HDMI to a HDMI-to-Minidisplayport adaptor.
I think that I have ruled out the drive being un-bootable as I just used it to do a fresh macOS install on another machine. Any advice on next steps would be greatly appreciated.