Startup Disk no longer works in Sierra long time lurker first time poster (go easy on me...)

I've had this problem since El Cap &amp; its finally driven me crazy enough to post here.

On my 2009 Mac Pro I had 2 drives. 1 with Yosemite &amp; 1 with Windows 8.1 under Bootcamp. I was able to use the Startup Disk Tool to select the Windows drive, hit restart, &amp; boot into Windows no problem. When I upgraded to El Cap, that process no longer worked. I would use the same process, hit restart &amp; my Mac would reboot, land on the gray screen, hang there for a bit, then go to a black screen &amp; bring up the prompt "NO BOOTABLE DEVICE - INSERT BOOT DISC AND PRESS ANY KEY". At which point I have to do a hold the power button down &amp; restart the system that way.

I tried reinstalling Windows, reinstalling El Cap, opening up the EFI partitions of both disks &amp; placing the various directories there, &amp; all sorts of other methods all to no avail.

I recently picked a 2010 Mac Pro tower &amp; hoped that the problem would be solved with a new install of Sierra &amp; Windows 10. No luck. It does the exact same thing.

I can hold Option while booting, select my Windows disk, &amp; proceed to Windows without issue. However, I much prefer the Startup Disk method (It lets me use a non flashed Nvidia card since I don't need to see anything before the system starts).

I suppose my question is 2 fold. 1) Is holding Option + selecting disk the canonical method for booting into Windows &amp; I'm wasting effort trying to get the Startup Disk method to work? 2) Has anyone gotten the Startup Disk Tool to work on their system &amp; if so can you give me any pointers?

Thanks for any help you can offer.
Not sure I understand your question. Do you wish to change the default startup disk?


  I can hold Option while booting, select my Windows disk, &amp; proceed to Windows without issue.


On the boot selection screen, instead of pressing enter, hold down control (the arrow will change) and then press enter at the same time. This will now be your new default.