Apple - How to update latest MacBook pro EFI firmware manually?
The EFI partition is used when the firmware is updated. I assume this EFI partition must be on the primary internal disk. Apple posted that firmware updates require a GUID partition scheme at the archived website Firmware updates for Intel-based Macs require a GUID partition scheme, although the site does not explicitly say this must be the primary drive. When a SDD or HDD is formatted to use a GUID partition scheme by the Disk Utility or diskutil command, the EFI partition is automatically created.
Basically, the firmware update is copied to the FAT32 formatted EFI partition. The firmware is then instructed to update itself. Basically, the same procedure occurs for firmware updates on my HP PC running Windows. So, this is not unique to Apple Macs.
Having to update the firmware to run High Sierra makes sense. The APFS is new and older Macs would need a firmware update before being able to recognize this filesystem and read the boot files.
You used to manually update the firmware. At some point, Apple decided to include firmware updates in the macOS installs. Apple posted this in the now archived website About EFI and SMC firmware updates for Intel-based Mac computers.
Just wanted to post on this topic since I had a related problem in an Apple store yesterday and this was the post I found.
Apple had replaced my logic board but didn't install the newer firmware and so I could no longer boot off of my existing SSD drive (which was formatted with APFS, and had my original Mojave and all of my files).
Luckily one of the geniuses had a good idea. Boot off of Mojave on a USB, and plug in a new blank external hard drive, and install Mojave onto that. As part of the installation process, the firmware was installed. Then, the USB stick and external hard drive were removed, the machine rebooted, and it could finally boot off of its original Mojave on the existing SSD drive.
Key point: Get the primary internal disk work.
Then unplug the second internal disk. Boot the machine with an USB to install high sierra, by following the standard installation guide. After that, the firmware will be upgraded.
Alternatively, you may use bless to update it manually, as long as the primary internal disk is there.