Open a Raspberry Pi SD card on a Mac

bootfs and rootfs: why only one shows up

A Raspberry Pi OS card holds two partitions. The first, bootfs, is a small FAT32 partition with the firmware, the kernel and the boot settings; on the running Pi it is mounted at /boot/firmware (before Bookworm, at /boot). The second, rootfs, is ext4 and holds everything else: the system, the programs, and your home folder.

macOS reads FAT, so bootfs mounts the moment you insert the card. It has no ext4 driver, so rootfs simply doesn't appear – without a word, because macOS complains only about a disk on which it can mount nothing at all.

A card or drive with only a Linux partition on it – a USB data drive from a Pi, say – does bring up “The disk you attached was not readable by this computer”. Don't click Initialize: here's what the message means and what to do instead.

Raspberry Pi OS formats rootfs as ordinary ext4 with a journal and extents; its image builder turns off only the 64bit and huge_file features. That is ext4 Extent reads, and writes once writing is unlocked.

Where things are on the card

Old guides that put a wpa_supplicant.conf on bootfs to set up Wi-Fi no longer apply: Raspberry Pi OS stopped reading it with Bookworm.

Reading the card on a Mac

1. 1

   Shut the Pi down, then take the card out

   sudo poweroff, or Shutdown in the desktop's menu, and wait until it has stopped. Raspberry Pi warns that cutting the power can corrupt the card. If the Pi won't start any more, just take the card out – see “A card pulled out without shutting down” below.

2. 2

   Install Extent and turn on its extension, once

   Extent is free on the Mac App Store. Turn it on in System Settings › General › Login Items & Extensions › File System Extensions; the banner in Extent's window opens that pane for you.

3. 3

   Insert the card

   In the Mac's SD slot or a USB card reader. bootfs mounts as it always did, and rootfs appears beside it in the Finder, read-only. If rootfs doesn't appear, the card probably went in before the extension was on: eject it and insert it again.

4. 4

   Copy what you need, then eject

   Drag files out as from any disk. Eject the card in the Finder before you take it out.

Editing files on the card

Files on bootfs – config.txt, cmdline.txt, an ssh file – need nothing extra: macOS writes FAT itself. Files on rootfs need writing turned on:

1. 1

   Unlock writing

   A one-time purchase, under Extent › Unlock Writing…. It covers every disk and every Mac signed in to your Apple Account.

2. 2

   Turn it on for the card

   Select rootfs in Extent's window and turn on “Write to this disk” in its details. Writing is a switch per disk, so a card you only read can't be changed by accident.

3. 3

   Eject the card and insert it again

   A file system is mounted either read-only or read and write; the switch takes effect the next time the card is connected.

4. 4

   Edit, then eject

   Save your changes as on any disk, and eject before you take the card out. Every change goes through rootfs's journal, as on Linux.

• A new file or folder takes the owner and group of the folder it's created in, so a file you add to /home/<username> belongs to that user on the Pi – not to a user ID that exists only on your Mac.
• macOS keeps a folder of its own, .fseventsd, on any disk it writes to, and the Finder may leave .DS_Store files in folders you open. Linux ignores both.

A card pulled out without shutting down

Linux writes every change to rootfs's journal first and moves it to its final place afterwards. A card taken out of a running Pi – or one whose Pi lost power – can hold changes that reached the journal and never their final place; the file system is marked as needing recovery. Linux finishes them, replaying the journal, the next time it mounts the card.

Software that reads the card elsewhere has to replay the journal too, or it shows the files as they were before those changes. Extent does:

• Read-only, as a card mounts by default, Extent replays the journal in memory: you see the files as Linux will once it has finished, and nothing is written to the card. Its window says “Linux didn't finish writing to this disk – it wasn't ejected properly.”
• With writing on, Extent replays the journal onto the card as it mounts it, as Linux would at the next boot, and finishes what the interruption left half-deleted.
• If the journal itself can't be read, Extent says the last changes made on Linux may be missing. If Linux had recorded damage on the card, Extent mounts it read-only and won't write to it until it has been checked with e2fsck -f on Linux.

Extent's replay was tested on a real card pulled out of a running Pi, with 274 transactions waiting in its journal.

A card that has started to fail – a Pi that keeps crashing, read errors – is best copied to an image once, before anything else, and the image opened instead. See the next section.

Back up the card as an .img, and open it

A copy of the whole card, both partitions, in one file the size of the card. In the Terminal:

1. 1

   Find the card

   diskutil list shows it as, say, /dev/disk4: a disk the size of the card, with a Windows_FAT_32 partition named bootfs and a Linux partition. Check the number twice.

2. 2

   Unmount its partitions, without ejecting

   diskutil unmountDisk /dev/disk4

3. 3

   Copy the card into a file

   sudo dd if=/dev/rdisk4 of=$HOME/Desktop/raspberrypi.img bs=4m status=progress – rdisk4 is the same card, read as a raw device. It takes as long as reading the whole card.

4. 4

   Eject the card

   diskutil eject /dev/disk4

5. 5

   Open the image

   In Extent, File › Open Disk Image…, or drop the .img on its window. macOS attaches it like a card: bootfs mounts by itself and rootfs through Extent, with the same folders as on the Pi.

diskutil list
diskutil unmountDisk /dev/disk4
sudo dd if=/dev/rdisk4 of=$HOME/Desktop/raspberrypi.img bs=4m status=progress
diskutil eject /dev/disk4

The image's rootfs has the card's file system UUID, and Extent keeps the writing switch per file system, so with writing on for the card the image can mount writable too. Turn it off in the image's details to keep the backup as it was.

On the Pi itself, Raspberry Pi OS with the desktop has SD Card Copier, which clones the card to another card or USB drive.

What Extent doesn't do

• It doesn't write Raspberry Pi OS to a card or format one – Raspberry Pi Imager does.
• It doesn't repair a damaged rootfs. That is e2fsck's job, on Linux – another Pi with a USB card reader will do; when a card needs it, Extent says so and gives you the command.
• It leaves bootfs to macOS, which reads and writes FAT itself.
• It runs on macOS 27 or later on a Mac with Apple silicon only. For other Macs and other ways to read ext4, see how to read and write ext4 on a Mac.

Frequently asked questions