Unable to use Boot Camp to switch boot device between MacOS/Windows 10 I have a Mac Pro 4,1 upgraded to a 5,1 with MacOS installed on an SSD and Windows 10 installed via a hacked Boot Camp on a separate HDD.

I'm running a GTX680 GPU which is Mac OS flashed to allow the display of the Mac boot screen over HDMI, but as I run a displayport connection, this is not that convenient when I want to switch from one OS to another.

I can use the Boot Camp option under both MacOS and Windows to choose by boot drive, but the Mac will always boot to Windows unless I switch to an HDMI connection and use the alt-key on reboot to choose my startup drive.

The SSD with MacOS on it is connected to the second optical drive SATA connection; the Windows drive is in the first drive bay. 

Any idea why choosing the boot drive via Boot Camp is not working?
You could try creating a USB flash drive with rEFInd installed. Make this your default boot drive. Next, see if rEFInd can be used to boot both operating systems. If so, then you could install rEFInd on your HDD or SSD to silently boot macOS.