"The disk you attached was not readable by this computer": recover your files first

You plug in a drive or card and macOS says: "The disk you attached was not readable by this computer," with Initialize, Ignore, and Eject. This is one of the few moments in data recovery where a single click makes the difference between an easy recovery and a hard one. Click Ignore. Not Initialize. Initialize opens Disk Utility so you can erase the disk, and Disk Utility will helpfully offer to do it. Erasing writes a fresh, empty filesystem over the one that holds your files.

The error means macOS found a device but couldn't mount a filesystem it recognizes on it. Your files are almost certainly still there. What follows is how to get them without making it worse. The short version is also on our help page.

macOS tried to read the partition map and the filesystem's superblock or volume header and either got an error or found something it doesn't understand. There are four common reasons, in rough order of likelihood:

The filesystem is one macOS can't mount. A drive formatted ext4 (Linux), or NTFS with an unclean shutdown flag, or a card from a camera that uses an unusual layout, will trigger this on a perfectly healthy device. Nothing is broken; macOS simply lacks the driver.

The filesystem is damaged. A cable pulled mid-write, a power loss, or a card removed while the camera was still saving can corrupt the small structures at the start of the volume (APFS container superblock, HFS+ volume header, exFAT boot sector, FAT32 boot sector). The rest of the volume, where your files live, is usually intact.

The partition map is damaged or gone. The disk has no readable GUID or MBR table, so macOS can't even find the volume. Files and filesystem are still in place, just unlisted.

The drive is failing. Bad sectors at the front of the disk, a struggling controller, or a USB bridge that's on its way out can produce read errors exactly where macOS needs to read first. This is the case that gets worse the longer you keep trying.

Open Disk Utility (Applications → Utilities) and choose View → Show All Devices. You'll see the physical disk and any volumes macOS found under it. Note the disk size. If the size is correct and a volume appears (greyed out, unmounted), you have a filesystem problem. If the disk shows the wrong size (a 2 TB drive showing 0 bytes or a few hundred KB) or doesn't appear at all, you have a hardware or bridge problem.

Do not click First Aid yet. First Aid attempts to repair the filesystem by writing to it. On a mildly damaged volume it often works. On a badly damaged one it can make structures unrecoverable that a read-only scan would have parsed fine. The rule: recover first, repair the volume second, when you have your files somewhere safe.

Also try a different cable, a different port, and a different reader or enclosure if you have one. A surprising share of "not readable" drives are readable through a different USB bridge.

If Disk Utility takes ages to show the device, the drive makes clicking or repeated spin-up sounds, or the Console shows I/O errors, stop scanning the live drive. Every retry on a failing disk costs you. Instead, make a byte-to-byte image: DataRecover Pro's Create Image reads the entire device once, front to back, skipping and logging bad regions instead of retrying them into the ground, and writes the result to a file on another drive with enough free space (the image is the size of the source). Then you scan the image, as many times as you like, with the failing hardware unplugged.

If the drive is clicking rhythmically or isn't detected at all in any enclosure, don't image it. That's mechanical failure and a recovery lab with a clean room is the right call. Software can't help a drive that can't read its own platters.

DataRecover Pro attaches to the disk read-only whether or not macOS could mount it. It reads APFS, HFS+, exFAT, FAT32 and NTFS directly, so a volume macOS can't mount is often one the app can still parse. Start with a quick scan. It reads the filesystem structures itself, and if the damage is limited to the boot sector or volume header, it will present your files with original names and folders in a minute or two.

If the quick scan finds nothing, or the partition map is gone, run a deep scan. Deep scan starts by searching the raw disk for lost partitions (it looks for the superblocks and boot sectors of every supported filesystem, anywhere on the device) and parses any it finds. Then it carves files by signature across 50 formats for anything the filesystem no longer references. On APFS it also walks older checkpoints of the container, which can surface a consistent earlier version of the filesystem when the current one is corrupt.

Preview what it finds: thumbnails and Quick Look are free with no account, so you can confirm your files are intact before paying. Recover to a different drive than the one you're recovering from. Download DataRecover Pro and run the scan before you let anything write to the disk.

Only now is it reasonable to try First Aid on the volume, and if that fails, to erase and reformat the disk (Initialize was right all along, just not first). If the drive threw read errors, don't put it back into service; a drive that has started failing does not stop. If it was a card, retire it. Then copy your recovered files to a second location before you trust any single copy.

If the drive isn't detected at all in Disk Utility's device list under any cable or enclosure, there's nothing for software to read; that's a lab job. If the disk was encrypted (FileVault APFS or an encrypted exFAT/NTFS tool) and you don't have the passphrase, the scan will find encrypted blocks and nothing else; DataRecover Pro can unlock APFS with your passphrase but can't guess one. And if someone already clicked Initialize and erased the disk, the filesystem is gone but the data mostly isn't; skip straight to the deep scan and expect carved files without original names. That case is covered in recover a formatted external drive on a Mac.