macOS Mojave installer killing Windows 10

[mikeboss]

lab/testing/Windows machine: Mac mini 2014 equipped with 2 SSDs (Apple/Samsung in the M.2 socket, Samsung S-ATA in the HDD bay). Windows 10 is installed on its own device (Samsung SSD in the HDD bay). the first time it happened was after installing one of the macOS Mojave 10.14.2 developer previews. now that 10.14.2 was released I did a fresh install of macOS onto the Apple SSD. and again, afterwards Windows 10 won't show up in the boot-picker anymore (holding option during power-up). really annoying...

 

[mikeboss]

ok, I think I know what was going on: I installed Windows 10 in EFI mode, not BIOS emulation mode onto disk1. macOS Mojave was on disk0. it seems that the Windows installer wrote some stuff to the EFI partition on disk0. of course, before doing a fresh install of macOS, I wiped disk0 clean (dd if=/dev/zero of=/dev/rdisk0). doing so, I deleted the EFI boot stuff of Windows 10. lesson learned: will install Windows in BIOS emulation mode in the future...