Yes, when I installing Windows, I removed Pcie SSD.
Windows I installing in legacy mode (booting from Windows, nori from efi) from DVD. SSD sata disk cleaned in Windows cmd diskpartition (in Windows startup, shift + F10). cleaned ant converted SSD in mbr and I dont use any format (fat and ntfs. Just left emty SSD).
Bootcamp 5.1 and 6.1 downloaded with Brigadier. And installed fully bootcamp 5.1 (disable automatic start up boot camp, for Windows desktop crash) and removed. Then fully installed boot camp 6.1 (and enable automatic start up boot camp) and then update manualy all drivers. Then insert Pcie SSD and stuck in Apple logo when booting Windows.
I try only install boot camp 5.1 (just disable automatic start up, for desktop crash) and insert Pcie SSD. Windows boot without ant crash (just normaly). Then unistall boot camp 5.1 (just left install drivers) and install boot camp 6.1. Windows charsh on Apple logo.
Then I try install boot camp 6.1 on renistall Windows. Insert Pcie SSD, crash in Apple logo. Unistall boot camp 6.1 (left only drivers) still crash in Apple logo.
I thing somthing wrong with boot camp 6.1 drivers.
I don't see an Apple logo when I start up. I think that was removed in later boot rom versions? I assume it is actually booting windows, but I can't say for sure since mine briefly shows a flashing text cursor in the upper left corner, and then immediately shows the Windows 10 logo.
In any case, if it really is trying to boot into windows, and if you keep force restarting the computer, it may eventually offer-up the windows recovery mode and then you can start up in safe mode. Once in safe mode, you can make sure your drive is MBR. https://www.easyuefi.com/resource/check-mbr-or-gpt.html
I did an install once, clearing all the partitions, and it converted my drive to GPT (UEFI). From then on, I wasn't able to use the Startup Disk control panel in MacOS to select the windows drive. It would just reboot to "No boot volume found" or something like that.
One thing that caused mine to crash on boot was Windows 10 insisted on installing GT 250 graphics drivers, and they will hang the computer. Since you said you have an ATI card, I can't imagine that is the issue. On the second try, I installed TightVNC so I could remote to the computer, even if there was nothing on the display.