i was able to revive mine (a different one - mid 2014 mbp with retina) by disconnecting the io board with hdmi, sd card slot, usb and wifi. apparently the reason is a conflict between firmware (?) and that card (possible cause in my view is inability to initialize all the bootable devices - sd card and usb are among those). ill try to downgrade firmware - should get that board functioning again…
i was able to revive mine (a different one - mid 2014 mbp with retina) by disconnecting the io board with hdmi, sd card slot, usb and wifi. apparently the reason is a conflict between firmware (?) and that card (possible cause in my view is inability to initialize all the bootable devices - sd card and usb are among those). ill try to downgrade firmware - should get that board functioning again…
Artem G.