Fix 'The disk can't be read from or written to' macOS

Quick answer for advanced users: Run Disk Utility First Aid on the affected disk and its container. If that fails, use Terminal to run fsck_apfs -l or diskutil verifyVolume from Recovery Mode.

Had a client last month whose entire external drive showed this error after a sudden power loss. She almost reformatted it and lost years of family photos. I stopped her just in time. This error pops up for a few reasons: a dirty eject, a failing cable, a dying drive, or a corrupted filesystem metadata. The OS throws the error because it can't trust the drive's structure enough to allow reads or writes. Don't panic yet — most of the time it's fixable.

1. Run First Aid in Disk Utility

This is the first thing to try. Open Disk Utility from Applications > Utilities, or search for it with Spotlight. Select the problematic disk in the left sidebar — not just the volume, but the physical disk (it'll show the brand and capacity, like "WD My Passport 1TB"). Click First Aid, then Run. Let it finish. It'll either report "overall status: healthy" or give you specific errors. If it succeeds, you're done. If it fails, move to step 2.

2. Boot into Recovery Mode and run First Aid from there

Sometimes macOS can't fix a disk while it's running because it holds a lock on some files. Restart your Mac and hold down Command+R (Intel) or keep pressing the power button until you see loading options (Apple Silicon). From the macOS Utilities window, choose Disk Utility. Run First Aid again on the same disk. Recovery Mode gives you a cleaner environment. I've seen this fix disks that looked completely dead under normal boot.

3. Use Terminal for a deeper fsck scan

If First Aid still fails, open Terminal from Utilities menu in Recovery Mode. Type diskutil list to identify your disk's identifier (like disk2s1). Then run:

fsck_apfs -l /dev/disk2s1

Replace the identifier with yours. The -l flag tells it to check for live containers. If you're on an older HFS+ disk, use fsck_hfs -f instead. Let it run — it can take 20-30 minutes on large drives. It'll output a lot of lines. If you see "The volume appears to be OK" at the end, you're golden. If it reports "File system check exit code 8", the drive might be physically failing.

4. Check the cable and connection

This sounds too simple, but I've burned 45 minutes on a bad USB-C cable more than once. Try a different cable, a different port, and — if it's an external drive — a powered USB hub or direct connection to a wall outlet (for powered drives). Some drives need more juice than a MacBook can provide through a single port, especially the older 2016-2019 USB-C models.

5. Try a different Mac or PC

Plug the drive into another computer. If that Mac reads it fine, the problem is your Mac's USB controller or privacy settings. If a Windows PC can read it (after installing HFS+ or APFS drivers), the disk is fine — it's a macOS issue. If no computer reads it, the drive hardware is likely toast.

Alternative fixes if the main steps fail

Data recovery first: If the disk is failing, don't keep hammering it with repair tools. Use Disk Drill or TestDisk to image the drive first, then run repairs on the image file.
Reset NVRAM/PRAM: On Intel Macs, shut down, then power on and immediately hold Option+Command+P+R for 20 seconds. This can clear weird disk-related flags.
Check Privacy settings: If you're on macOS Ventura or later, go to System Settings > Privacy & Security > Files and Folders. Make sure Disk Utility and Terminal have full disk access. I've seen this block repairs silently.
Remove third-party kernel extensions: Things like Paragon NTFS or Tuxera can interfere with disk mounting. Uninstall them temporarily and try again.

Prevention tip

Don't yank cables. Always eject drives properly (drag to Trash or right-click > Eject). For external drives, enable TRIM if it's an SSD (but only for Apple-approved drives — don't force it on third-party SSDs unless you're sure). Also, run First Aid once a month on any drive that holds important data. And for heaven's sake, have a backup. Time Machine is free and built in. If that disk dies completely, you'll wish you'd set it up.