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Install EXT4 brew install e2fsprogs. This will not add support to the Disk Utility, but it can be run.

Find my SD Card.

Setup BOOT partition.

Setup ROOT partition.

$(brew --prefix e2fsprogs)/sbin/mkfs.ext4 /dev/disk2s2

Install mkfs.ext4 brew install e2fsprogs. This will not add support to the Disk Utility, but it allow you to make an EXT4 partition. You will also need to mount it to extract the Arch Linux tar to it, so also get yourself brew cask install osxfuse && brew install ext4fuse.

Find SD Card Device Node

Setup BOOT partition.

Setup ROOT partition.

Caveat: I am using EXT 3 instead of EXT 4, the non-journaled variant, because the FUSE implementation does not support writing to journaled EXT yet, and I could not find the no_journal option mentioned here: https://github.com/gerard/ext4fuse/wiki/Write-Support. As soon as it gets implemented, an EXT 4 write on MacOS will be possible.

$(brew --prefix e2fsprogs)/sbin/mkfs.ext3 /dev/disk2s2
$(brew --prefix ext4fuse)/bin/ext4fuse /dev/disk2s2 /Volumes/ROOT -o allow_other

Indicates that MS-DOS FAT 32 is supported.

                      #/dev/node  PARTSCHEME FS LABEL SIZE (R=remainder, 100%)
diskutil partitionDisk /dev/disk2 MBR FAT32 BOOT 100M FAT32 ROOT R

macOS will automount the new partition at /Volumes/PI, so skip the mount step.

Unmount and you are good to go  (which probably flushes out the buffers anyway)

Indicates that MS-DOS FAT 32 is supported. But EXT 4 is not.

Install EXT4 brew install e2fsprogs. This will not add support to the Disk Utility, but it can be run.

Setup BOOT partition.

                       /dev/node  PARTSCHEME FS LABEL SIZE (R=remainder, 100%)
diskutil partitionDisk /dev/disk2 MBR FAT32 BOOT 100M FAT32 ROOT R

macOS will automount the new partition at /Volumes/BOOT, so skip the mount step.

Setup ROOT partition.

$(brew --prefix e2fsprogs)/sbin/mkfs.ext4 /dev/disk2s2

Unmount and you are good to go. (which probably flushes out the buffers anyway)

diskutil unmount /Volumes/{BOOT,ROOT}

# unmountDisk unmounts both ROOT and BOOT
diskutil unmountDisk /Volumes/ROOT