@TheOtherAMF mm. my problem isn't about booting into an OS, but that Win10 doesn't want to see this android phone when the phone's in fastboot mode. therefore i can't flash it or do anything else useful from windows.

linux to the rescue, it seems.

my early i7 (LGA-1366) mobo is from 2010? so no UEFI to worry about. i hope it's not too awful whenever i "upgrade" to a more modern system.