“The disk you attached was not readable”: when it’s a Linux disk

What the three buttons do

The message comes with three buttons. Only one of them can cost you the files:

Older versions of macOS said “The disk you inserted was not readable by this computer”. It is the same message.

Many guides for this message offer data-recovery software, First Aid or reformatting. For a disk that came from Linux, none of that is needed: nothing is wrong with the disk, macOS just can't read its format.

When macOS shows it

macOS reads APFS, Mac OS Extended, FAT and exFAT, and NTFS for reading. When it can mount nothing at all on a disk – no partition with a file system it knows – it shows this message. One readable partition is enough to keep it quiet.

That is why a Raspberry Pi's SD card doesn't show it: the card's small FAT boot partition mounts, and the ext4 partition beside it, with the system and your files, silently doesn't appear. Opening a Raspberry Pi card on a Mac covers that case.

How to tell it's a Linux disk

Choose Ignore, open the Terminal and list the disks:

diskutil list

Find the disk by its size. A Linux disk shows Linux in the TYPE column of its partitions – for example a USB drive formatted on Ubuntu:

/dev/disk4 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *64.0 GB    disk4
   1:           Linux Filesystem                         64.0 GB    disk4s1

The TYPE is what the partition was marked for, not what is in it. Some tools format ext4 inside a partition marked Microsoft Basic Data, and a disk formatted as a whole, without partitions, shows no TYPE at all. Extent looks at the file system itself, and since version 1.0.2 it lists every Linux partition it can't open with what it found there – LVM, LUKS, RAID, Btrfs, XFS – so its window answers the question too.

Opening it on the Mac

1. 1

   Choose Ignore or Eject

   Either leaves the files where they are.

2. 2

   Install Extent and turn on its extension, once

   Extent is free on the Mac App Store. Switch 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

   Disconnect the disk and connect it again

   macOS offers a disk to the installed file systems when it is connected. One that was connected before the extension was on isn't offered again until it is reconnected.

4. 4

   Open it in the Finder

   An ext2, ext3 or ext4 disk now mounts by itself, read-only, and the message doesn't come back. To change files on ext4, writing is a one-time purchase and a switch per disk – see how to read and write ext4 on a Mac, which also compares the other ways to do it.

If the message comes back with Extent installed

Extent's window says why, in a sentence, for every Linux partition it couldn't open:

• The extension is off. A banner says so, with a button to the switch.
• The disk was connected before the extension was on, or while the Mac was starting up. Eject it and connect it again.
• It holds LVM, LUKS, a software RAID, Btrfs or XFS. Extent names what it found and can't open it; anylinuxfs can, or the Linux computer the disk came from. Which disks Extent opens, device by device.
• The ext file system is damaged – a checksum that doesn't match, a file system larger than the disk. Extent leaves it alone and tells you to check it with e2fsck on Linux.
• Another file system driver mounted it – Paragon's, or another ext4 app. Extent names the driver.

When it isn't a Linux disk

If diskutil list shows no Linux partition, Extent can't help, and the cause is elsewhere:

• A new disk that was never formatted. Then Initialize is the right button – or, for a disk meant for Linux, format it as ext4.
• A disk encrypted with BitLocker on Windows, or one a camera, TV or recorder formatted its own way. macOS reads neither.
• A cable, hub or card reader that doesn't pass the disk through properly. Try another port or reader before anything else.

Whatever the disk is, don't erase it while the files on it matter and you don't yet know what is on it.

Frequently asked questions