"The disk you attached was not readable by this computer"

The message “The disk you attached was not readable by this computer” means macOS could not make sense of the file system on the drive or card, the index that records where each file lives. It does not mean the files are gone. Click Eject or Ignore, do not erase anything, and stop recording to the card. Then find out whether the cause is the connection, a format the Mac does not read, or a damaged file system, because each has a different answer.

What the three buttons do

• Eject disconnects the disk. Nothing is changed. This is the safe choice.
• Ignore leaves the disk attached but not mounted. Nothing is changed, and Disk Utility can still see it.
• Initialize opens Disk Utility so that you can erase the disk and set it up fresh. Nothing is erased until you click Erase there, but erasing is the only place that path leads. It is right for a new, empty drive and wrong for a card full of photos.

If the card came out of a camera, refuse the camera’s own offer to format it as well. On a full-size SD card, slide the lock tab on its side to the locked position before trying anything else, so that nothing can write to it.

Rule out the connection

Many unreadable disks turn out to be readable through a different path.

1. Take the card out of the reader, check the contacts for dirt, and seat it again.
2. Try a different reader, a different cable, and a port on the Mac itself instead of a hub. Portable hard drives in particular misbehave on hubs that do not supply enough power.
3. Consider the reader’s age. Cards larger than 32 GB are SDXC cards, and a reader made only for the older SD and SDHC types may not read them.
4. Put the card back in the camera, with the lock tab still on, and see whether the camera can show the pictures. If it can, connect the camera to the Mac with its USB cable and import with Image Capture. The camera reads its own card for you.

See what the Mac can detect

Open Disk Utility and choose View, Show All Devices. Look for the card or drive in the sidebar.

• It appears with the right capacity, and its volume is dimmed or missing. The hardware is responding and the file system is the problem: it is damaged, or it is a format macOS does not read.
• It appears with the wrong capacity, such as a few megabytes. That points to failing hardware or a bad reader.
• It does not appear at all. The Mac is not detecting the device. Go back to the connection checks, and try another computer.

If the format is foreign, nothing is wrong

Some disks are unreadable on a Mac by design:

• Drives formatted by Linux systems, which includes many network storage boxes.
• Drives from video recorders, security systems, televisions and game consoles, which often use formats of their own.
• Drives encrypted by another operating system.
• A new drive that has never been formatted.

Read these on the device that made them, and copy the files across the network or onto a drive formatted as exFAT, which both Macs and Windows PCs can read and write. A new, empty drive is the one case where Initialize is the right button.

If the file system is damaged

This is the usual story with camera cards. The card was pulled out while the camera was writing, the battery died in the middle of a recording, or the card was removed from a computer without being ejected. The photos and videos are normally still on the card. What is broken is the index.

Take these steps in order, and do not skip the first when the files matter.

1. Copy the whole card to an image file. A repair writes to the card, and on a failing card every extra operation is a risk. An image is a byte-for-byte copy you can work on instead. In Terminal, find the card’s identifier with diskutil list (for example disk4), make sure the Mac has as much free space as the card’s capacity, and run:

sudo dd if=/dev/rdisk4 of=~/Desktop/card.img bs=1m conv=noerror,sync

Read the command twice before pressing Return. if= is the card you are reading and of= is the new file on the Mac; swapping them overwrites the card. It asks for your Mac password and prints nothing while it works, which can take a long time for a large card.

2. Run First Aid. Unlock the card’s tab, since a repair has to write. In Disk Utility, select the dimmed volume or the device and click First Aid. If it repairs the file system, the card mounts and you can copy the files off in Finder. If it reports that it cannot, you still have the image.

3. Scan with file recovery software. Recovery tools ignore the broken index and search the raw data for photos and videos. Point one at the image file if it accepts images, or at the card, and always save what it finds to a different drive.

4. For irreplaceable files on failing hardware, stop and use a data recovery lab. Repeated attempts on a dying card reduce what a lab can get back.

Once everything you need is copied and checked, format the card in the camera, not on the Mac. Retire a card that becomes unreadable a second time.

Where Manatee fits, and where it does not

Manatee does not repair disks, and it is not an undelete or recovery tool. It cannot read a card that macOS cannot read, and it is not the first thing to reach for here.

It becomes useful one step later. Files that come back from a damaged card are often incomplete: a video that will not open because its index was never written, or a document that stops partway through. Manatee’s Repair mode works on video, PDF and Word documents. It rebuilds what is readable and reports the result plainly as Fixed, Partial or Not fixable. It writes a new copy beside each file and never modifies the original, and nothing leaves the Mac. It is free for 24 hours with every feature, which is usually long enough to learn what can be saved, then $9 once.

Recovered videos that will not play after an SD card rescue covers that stage in detail, and why files get corrupted explains what is and is not recoverable.

Questions

Does this message mean my files have been deleted? No. It means the Mac could not read the structure that organizes them. With a damaged file system the files are usually still there, and with a foreign format they are untouched.

Is it safe to click Initialize? Clicking it only opens Disk Utility. The erase happens if you then click Erase, and that removes the index for good and makes recovery harder. Choose it only for a disk you intend to wipe.

The card works in the camera but not on the Mac. Why? Usually the reader or the cable is at fault, so try another. Otherwise the camera may be tolerating damage that macOS will not. Import through the camera’s USB connection while it still works, then format the card in the camera.