Unable to install Windows 10 via USB boot disk

I'm trying to install Windows 10 via Boot Camp. I've followed the Boot Camp instructions to the letter - USB drive erased & formatted as ExFAT with no partitions, used Boot Camp to copy ISO & drivers to the USB drive, created BOOTCAMP partition via Boot Camp, & allowed Boot Camp to restart the computer. But when it restarts, I get a "No Bootable Device - Please insert boot disc and press any key." error (and pressing any key does nothing). I can restart and hold down Option key and get the correct Mac, Windows, and WININSTALL disk options. I can boot into the install disc and start the install process ("Custom" to install from scratch), but when I get to the point where I choose the partition, it gives me a "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." error and I can't continue. I tried erasing/formatting the partition via the Windows utility, but I still just get the same error.

I'm on a 13-inch, mid-2012 MacBook Pro (9,2) with the latest Sierra updates. From my internet reading, supposedly I shouldn't even be able to install from the USB, but that's the only option Boot Camp gives me.

I have nothing else plugged in - just the power supply and USB boot drive.

Help? 🙂