No bootable device - bootcamp Windows 10

Hello everyone,

I've seen the problem in the discussions but the answers don't seem to help me. I'll quickly explain:

MBP mid '12, 512 HDD and 8 RAM on macOS Catalina and I try to install Windows 10 64bit through Boot Camp assistent.

I went through the Boot Camp assistant, and it all went smoothly. It installed the ISO image onto my Buffalo HDD 2TB and again on my Toshiba 500GB. Then, the computer restarted.

When the MBP restarts, it shows the normal white screen, and then it shows a black screen that says "No bootable device -- insert boot disk and press any key". I am not sure why this happened.

So next I try to circumvent this issue by restarting, holding Alt/Option, and it brings up the boot order page. The options are "Macintosh HD", "Windows", and "EFI". I select the Windows option. It brings me back to my original problem with the black screen and the "no bootable device" message.

Next, I try to instead start up with the EFI option. This successfully enters the Windows setup, but when I progress through all of the steps, at the step where I select which partition to install Windows on, it tells me that "Windows cannot be installed to this disk. The selected disk has an MBR partition table. On EFI systems, Windows can only be installed to GPT disks." Now, I sort of knew that this was going to come up because Mac usually uses an MBR partition, but it is arranged in a BIOS system for Windows to use. If I use EFI, like I selected before, it will want a GPT disk format.

I read somewhere this problem is due using a USB3 instead of USB2 but I'm 99% sure I only have USB2's. (I was told to check the speed = 480 Mb/s)

Any ideas? (might it be Windows 32bit vs 64bit?)

Thanks in advance