That Windows 10 boot issue

Hi! I’ve got a mid-2012 Mac Pro with a Windows 10 installation, and a pair of macOS installs as well. I had the same issue a couple other people have, an inability to boot Windows; attempting to do so from the Boot Runner menu or CLI would result in a reboot, followed by a return to macOS. I’ve got a Radeon RX 580 in the thing, so the only way I have to make use of the option menu on boot, rEFInd, or Next Loader is to swap in a slower card, which would naturally make Boot Runner by far the least inconvenient way to switch operating systems.

I’ve done a little simple instrumentation of /sbin/reboot and /usr/sbin/bless, and I suspect I know what’s going on.

Windows won’t boot happily with EFI, presumably because I’ve got an older machine, so it requires a legacy boot. Boot Runner, from the logs, appears to realize this, even without a profile, but I forced one of each boot types anyway to check things out.

default, Legacy, and EFI all call bless with:
/usr/sbin/bless -device /dev/disk1s2 -setBoot -nextonly
(disk1s2 being my bootcamp device).

Oddly, Volume EFI calls bless with:
/usr/sbin/bless -device /dev/disk1s2 -setBoot -nextonly -nextonly

…which throws an ‘Option “nextonly” already specified’ error and gives a usage message.

None of these actually specify -legacy anywhere, and if there’s another parameter I can add (to an individual volume config, say) to force that, I haven’t run across that, but adding it manually from the command line:
/usr/sbin/bless -device /dev/disk1s1 -setBoot -nextonly -legacy

…followed by a reboot works as expected.

I’ll refrain from explaining my interception in detail here, as it could encourage the unwary to engage in dangerous behavior; likewise, I could probably install an awful kludge to add -legacy to the command line, but that would be a little gross.

Anyway, hope this helps some. Sorry for the long blather.

Boot Runner was doing fine when I had 3 drives: Mojave, High Sierra and Carbon Copy Cloner and would boot from any one of those. I have installed a separate drive with W/10 which BR sees and displays the Windows logo on the disk ikon. BUT if I select W/10 to boot it just reboots Mojave. If I pull all the drives except W10 then Windows opens fine but if I rely on BR it does not? Any help out there?