long time lurker first time poster (go easy on me...)
I've had this problem since El Cap & its finally driven me crazy enough to post here.
On my 2009 Mac Pro I had 2 drives. 1 with Yosemite & 1 with Windows 8.1 under Bootcamp. I was able to use the Startup Disk Tool to select the Windows drive, hit restart, & boot into Windows no problem. When I upgraded to El Cap, that process no longer worked. I would use the same process, hit restart & my Mac would reboot, land on the gray screen, hang there for a bit, then go to a black screen & 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 & restart the system that way.
I tried reinstalling Windows, reinstalling El Cap, opening up the EFI partitions of both disks & placing the various directories there, & all sorts of other methods all to no avail.
I recently picked a 2010 Mac Pro tower & hoped that the problem would be solved with a new install of Sierra & Windows 10. No luck. It does the exact same thing.
I can hold Option while booting, select my Windows disk, & 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 & 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 & if so can you give me any pointers?
Thanks for any help you can offer.