MacOS – How to create “firmware partition” on HDD

You can't partition disk1 freely because it contains a Core Storage Volume Group (disk1s2 -> disk2).

You can revert to a non-CoreStorage by booting to Macintosh HD (disk0s2) and opening Terminal.

Enter the following to get all necessary infos:

diskutil cs list

Example listing:

      CoreStorage logical volume groups (1 found)
    |
    +-- Logical Volume Group A629E051-D7B0-4B8C-A803-074F62704636
        =========================================================
        Name:         System
        Status:       Online
        Size:         53946696192 B (53.9 GB)
        Free Space:   16777216 B (16.8 MB)
        |
        +-< Physical Volume 90C09FC0-4215-4871-901B-70E2C9C7D464
        |   ----------------------------------------------------
        |   Index:    0
        |   Disk:     disk0s2
        |   Status:   Online
        |   Size:     53946696192 B (53.9 GB)
        |
        +-> Logical Volume Family F6962E38-50E4-4458-BFE6-CF2E179352F5
            ----------------------------------------------------------
            Encryption Status:       Unlocked
            Encryption Type:         None
            Conversion Status:       NoConversion
            Conversion Direction:    -none-
            Has Encrypted Extents:   No
            Fully Secure:            No
            Passphrase Required:     No
            |
            +-> Logical Volume BD36C73D-860D-4DC6-B125-AD624F448B88
                ---------------------------------------------------
                Disk:                  disk2
                Status:                Online
                Size (Total):          53496696192 B (53.5 GB)
                Conversion Progress:   -none-
                Revertible:            Yes (no decryption required)
                LV Name:               System
                Volume Name:           System
                Content Hint:          Apple_HFS

If your Logical Volume is revertible enter:

diskutil cs revert lvUUID

(lvUUID is the UUID of the Logical Volume)

with the UUID of the example above

diskutil cs revert BD36C73D-860D-4DC6-B125-AD624F448B88

If the Logical Volume is encrypted you have to enter a passphrase.

After the conversion repartition the drive as needed.

If the LV isn't revertible you have to copy the content of the volume 21Tech to another volume and delete the Logical Volume Group with:

diskutil cs delete lvgUUID

(lvgUUID is the UUID of the Logical Volume Group)

with the UUID of the example above

diskutil cs delete A629E051-D7B0-4B8C-A803-074F62704636

Then repartition the drive as needed.

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.