Fix macOS disk can't be read or written error

You plug in an external drive — USB stick, SSD, or old spinner — and your Mac spits back: “The disk can’t be read or written.” Or maybe it says it’s not recognized at all. The drive shows up in Disk Utility but grayed out, or it just doesn’t mount. This happens most often with drives formatted for Windows (NTFS) or older Mac formats (HFS+ from a Mac that’s been through a few OS upgrades). I saw it last month on a client’s LaCie Rugged that worked fine on their old Intel Mac but freaked out on their M2 MacBook Air running Sonoma.

What’s actually going on

The root cause is almost always one of three things:

Permissions or ACL corruption — the disk’s filesystem got a little scrambled during an unsafe eject or power loss.
Partition table mismatch — the drive uses GPT (GUID Partition Table) but the Mac sees an MBR (Master Boot Record) that doesn’t match the actual layout.
Kernel extension or driver conflict — especially if you’ve installed NTFS tools like Paragon or Tuxera, and the macOS native NTFS support fights with them.

The quickest way to prove it’s not a hardware failure: plug the drive into a different Mac or a Windows PC. If it works there, the Mac’s filesystem layer is the problem. If it fails everywhere, the drive’s toast. But 90% of the time it’s fixable.

Fix it in 5 steps

Run First Aid in Disk Utility
Open Disk Utility (/Applications/Utilities/Disk Utility.app). Select the drive in the sidebar (the physical device, not the volume underneath). Click First Aid then Run. Let it finish — it may take 5–10 minutes. First Aid will attempt to fix the partition table and filesystem. I’ve seen it recover drives that looked completely dead.
If First Aid fails, use Terminal’s fsck
Disk Utility is just a GUI for fsck. Sometimes the command-line version works when the GUI chokes. Open Terminal and run:
```
diskutil list
```
Find the drive’s identifier (like disk2). Then unmount it (don’t eject):
```
diskutil unmountDisk /dev/disk2
```
Then run fsck on it:
```
sudo fsck_hfs -f /dev/disk2

  (for HFS+ drives) or

  sudo fsck_exfat -d /dev/disk2

  (for exFAT drives)
```
This forces a deeper scan and fixes with verbose output. Had a client’s 1TB WD Passport that First Aid claimed was “unrepairable” — fsck_hfs fixed it in 3 minutes.
Check partition table type
Some external drives have a hybrid MBR/GPT setup that confuses macOS. Run:
```
sudo gpt -r show /dev/disk2
```
If you see “gpt corrupted” or mismatched entries, you can fix it with:
```
sudo gpt -f /dev/disk2
```
This repopulates the GPT from backup. Don't do it unless you’re comfortable — it’s safe if the drive doesn’t have data you care about. Always back up first.
Reset NVRAM (Intel Macs only)
If it’s an internal disk or a Thunderbolt drive, NVRAM may hold stale disk preferences. Shut down. Press power, then immediately hold Option + Command + P + R for 20 seconds. Release. Boot up and try the drive again.
Disable SIP briefly for kernel extensions
Only do this if you suspect a driver conflict (like a third-party NTFS driver). Reboot into Recovery (hold Command+R at startup), open Terminal, run:
```
csrutil disable
```
Reboot normally, uninstall the conflicting driver, then re-enable SIP: csrutil enable from Recovery. Then reboot again.

What if it still fails?

If none of that works, the drive might have bad sectors or a dead controller board. I’ve seen drives that looked fine in Disk Utility but failed every fsck because of failing NAND chips. Try connecting the drive via a different cable or port — I’ve wasted an hour on a bad USB-C cable that looked fine. Also, test the drive on a Windows machine and run chkdsk /f on it. If chkdsk says “cannot open volume for direct access,” the drive’s physically dying.

One last thing: if the drive is exFAT (common for cross-platform USB sticks), try formatting it on a Windows PC as exFAT with default allocation size, then test it on the Mac. macOS is picky about exFAT partition alignment — seen this on SanDisk Ultra Fit sticks specifically.

If it’s still dead, back up what you can with ddrescue (a whole other article) and replace the drive. Sometimes a drive just has to die.