Read an ext4, btrfs or XFS drive on a Mac

Plug in a drive from a Linux machine and Finder offers to erase it. Here is why, and how to read it instead.

Why a Mac cannot read the drive

Every drive has a filesystem: the arrangement that records where each file starts, what it is called, and which parts of the disk are still free. Windows uses NTFS. macOS uses APFS. Linux usually uses ext4, and sometimes btrfs or XFS.

The files themselves are ordinary. What differs is the index in front of them, and a computer that cannot read the index has no way to find anything, even though everything is still there.

What happens when you plug one into a Mac

Finder says the disk is unreadable and offers to erase it. That is the same message macOS gives for a blank disk or a failing one, so the message gives you no way to tell the drive is fine.

macOS includes support for its own filesystems and for a few common ones such as exFAT. The Linux filesystems are not among them, so an ext4 drive looks to macOS like a blank disk.

How Lukotta reads it

Lukotta starts a small Linux system in the background and gives it the drive. Linux reads ext4, btrfs and XFS on its own, so the drive opens there without any conversion, and Lukotta passes it on to Finder.

Nothing is written to the drive, and it behaves normally when it goes back into the Linux machine.

What Lukotta reads

ext2, ext3 and ext4, which cover most Linux systems. btrfs, the default on Fedora and openSUSE. XFS, common on servers.

Encrypted or not. If the drive was set up with LVM, which divides one physical drive into several volumes, they all open together rather than one at a time.

Opening the drive

1. Plug in the drive. Lukotta lists what is on it.
2. If it is encrypted, type the passphrase.
3. The drive appears in Finder under Locations.

If it asks for a passphrase

LUKS is how Linux encrypts a whole drive. If the machine this drive came from asks for a passphrase early in startup, before the login screen, that is LUKS, and it is the passphrase to type here.

It is separate from the login password, though many people set both to the same thing.

Drives that will not open

A LUKS drive can be set up with its header stored elsewhere, on another disk or a USB stick. The header is the part that describes the encryption. Without it there is nothing on the drive to unlock.

Newer Ubuntu installations can also tie the key to a chip inside the machine, in the same way BitLocker does. Those drives only open in the computer they came from.