I repairs Macs at work, so I boot a lot of machines via external drives. I think some firmware updates will work via external drive and some won't.
I remember trying to get an iMac to install one, hoping it would fix an issue, and the internal drive was encrypted. I had to erase/reinstall on the internal drive to get the update to install. (Note to university computer lab admins: if your Thunderbolt isn't working, try installing the "Thunderbolt Firmware Update" that Software Update prompts you to install before requesting a new logic board ;-)
Firmware updates aren't uncommon, when a machine or architecture is new. I can remember firmware updates for EFI (added internet recovery when Lion came out), Thunderbolt, Magsafe/power, SSDs, and Wifi across all types of Macs.
I remember trying to get an iMac to install one, hoping it would fix an issue, and the internal drive was encrypted. I had to erase/reinstall on the internal drive to get the update to install. (Note to university computer lab admins: if your Thunderbolt isn't working, try installing the "Thunderbolt Firmware Update" that Software Update prompts you to install before requesting a new logic board ;-)
Firmware updates aren't uncommon, when a machine or architecture is new. I can remember firmware updates for EFI (added internet recovery when Lion came out), Thunderbolt, Magsafe/power, SSDs, and Wifi across all types of Macs.