macOS 'The disk can't be read' error fix

Quick answer

Open Disk Utility, select the disk (not the volume), and run First Aid. If that fails, use Terminal with diskutil verifyDisk /dev/diskX then diskutil repairDisk /dev/diskX. Replace X with your disk number (find it via diskutil list).

Why this error shows up

You plug in an external drive—maybe a Seagate Backup Plus or a WD My Passport—and instead of mounting, you get a dialog: “The disk you inserted was not readable by this computer.” Clicking Initialize or Ignore doesn't help. What's actually happening here is that macOS's kernel extension apfs.kext or hfs.kext can't read the disk's partition table—the GUID Partition Table (GPT) header got corrupted, or the filesystem superblock has a bad checksum. This doesn't mean the data is gone. The structural metadata is damaged, not the files themselves.

Common triggers: unplugging a drive without ejecting, a sudden power loss during a write, or a failing USB cable that causes intermittent disconnects while the drive is spinning. I've seen this happen on macOS Ventura 13.4 and Sonoma 14.2 with external SSDs and HDDs alike.

Fix steps (numbered)

1. Open Disk Utility — from /Applications/Utilities or search with Cmd+Space. View > Show All Devices (critical: you need to see the physical disk, not just volumes).
2. Identify the problematic disk — it'll appear grayed out or with a name like “disk2s2”. Click it on the left sidebar. The disk entry is the top-level item, often labeled with the manufacturer name and capacity (e.g., “ST1000LM048” or “WD Elements 2TB”).
3. Run First Aid on the disk — click First Aid in the toolbar, then Run. The reason step 3 works is that First Aid uses fsck_apfs or fsck_hfs under the hood to repair the filesystem. But if the partition table is damaged, First Aid on the volume level won't help—you must target the disk itself. Wait for it to finish. If it says “overall status: repaired”, you're done. Eject and reconnect.
4. If First Aid fails — it'll say “Operation failed” or “first aid process encountered a problem before finishing”. Don't panic. Close Disk Utility and open Terminal.
5. Find the disk identifier — run diskutil list. Find your external drive by size. It'll look like /dev/disk2 (external, physical). Note the number (2 in this example).
6. Verify the disk partition map — run diskutil verifyDisk /dev/disk2. The output will show “Invalid partition map” or “partition checksum mismatch”. That confirms the GPT header is corrupt.
7. Repair the partition map — run diskutil repairDisk /dev/disk2. This rebuilds the GPT header from the backup GPT table stored at the end of the disk. The reason step 7 works is that macOS keeps a second copy of the partition table in the last 34 sectors. If only the primary header is corrupt, the backup is still intact.
8. Re-run Disk Utility First Aid — after the repair, go back to Disk Utility and run First Aid on the volume now. It should mount cleanly.

If the disk still won't mount

Sometimes the GPT is fine but the filesystem itself has deeper corruption. Try fsck_apfs -l /dev/disk2s1 (replace disk2s1 with the actual volume identifier). The -l flag runs a live check without unmounting. If the volume is already unmounted, drop the -l. On APFS, use diskutil apfs verifyVolume /dev/disk2s1.

For HFS+ (Mac OS Extended) drives: diskutil verifyVolume /dev/disk2s1 and then diskutil repairVolume /dev/disk2s1.

If all Terminal commands fail with “The underlying task reported failure”, the partition table might be beyond repair. In that case, third-party tools like DiskWarrior or R-Studio can reconstruct the directory. But don't pay for anything until you've tried the free route: ddrescue if the drive has bad sectors, or simply connecting the drive to a different Mac or a Linux box.

Prevention tips

• Always eject before unplugging — drag to trash or use Cmd+E. This flushes the write cache and writes back the volume header.
• Use a short, high-quality USB cable — cheap cables with loose connectors cause voltage drops that make macOS think the drive disconnected and reconnected, corrupting the partition map.
• Check disk health quarterly — run smartctl -a /dev/disk2 (install smartmontools via Homebrew) to see reallocated sectors and pending errors. Catching a failing drive early saves your data.
• Back up everything — Time Machine to a separate drive. If you value your data, run a manual rsync every month. Drives die, partition tables corrupt. You don't want to be the person asking “can I recover this?” on Reddit.

One last thing: if you see “The disk can't be read” on your internal Mac drive (not external), stop and make a backup immediately. Internal SSD failures on T2-chip Macs are nasty—data recovery often requires Apple's hardware tool. Don't run First Aid on the internal drive unless you have a recent backup.