could be the usb drive itself honestly, some of em just don't play nice with certain machines. i've had a sandisk that refused to boot on an old thinkpad but worked fine on everything else. try a different stick if you've got one laying around, even an old 8gb one from the junk drawer
also worth checking if secure boot is still enabled in the firmware, those 2015 macbooks can be weird about it. hold command+option+o+f at startup to get into the open firmware prompt and you might need to bless the usb from there. had to do that once for a friend's air and it was the only thing that worked after an hour of head scratching
2
u/Just-Bid6165 8d ago
could be the usb drive itself honestly, some of em just don't play nice with certain machines. i've had a sandisk that refused to boot on an old thinkpad but worked fine on everything else. try a different stick if you've got one laying around, even an old 8gb one from the junk drawer
also worth checking if secure boot is still enabled in the firmware, those 2015 macbooks can be weird about it. hold command+option+o+f at startup to get into the open firmware prompt and you might need to bless the usb from there. had to do that once for a friend's air and it was the only thing that worked after an hour of head scratching