What's Happening
You plug in an external drive or USB stick. macOS pops up a dialog:
The disk you inserted was not readable by this computer.
Three buttons: Initialize, Ignore, Eject. Don't click Initialize — it wipes everything on the drive.
The error means macOS detected the hardware but can't parse the filesystem. Typical culprits: an unsupported filesystem (NTFS, ext4, or exFAT with a trashed partition table), a corrupted partition, or a flaky USB connection.
Quick Diagnosis
Start here before touching anything else:
diskutil list
If your drive shows up — say, as /dev/disk2 — the hardware is fine. The problem is at the filesystem level. Skip straight to Fix 1.
If it's not in the list at all, run this:
system_profiler SPUSBDataType
This dumps every USB device macOS sees. Drive appears here but missing from diskutil list? Partition table is the culprit. Drive absent from both? Hardware issue — jump to Fix 6.
Fix 1: Run Disk Utility First Aid
The drive shows in diskutil list? Try First Aid before anything else:
diskutil repairVolume /dev/disk2s1
Swap disk2s1 for whatever identifier you saw in the output. You can also do this visually: open Disk Utility → select the drive → First Aid → Run.
If it completes cleanly, eject the drive, unplug it, plug it back in. Should mount normally.
Fix 2: Force-Mount the Disk
Sometimes macOS sees the disk but just won't auto-mount it. Give it a nudge:
diskutil mountDisk /dev/disk2
Or target a specific partition:
diskutil mount /dev/disk2s1
Success looks like the drive instantly appearing in Finder.
Fix 3: NTFS Drives (Formatted on Windows)
Windows marks an NTFS volume as "dirty" if it wasn't shut down cleanly — think: yanked the cable, power cut, Windows crash. macOS sees dirty = unreadable.
First, confirm you're dealing with NTFS:
diskutil info /dev/disk2s1 | grep "File System"
Two ways forward:
- Option A — Fix on Windows: Plug the drive into any Windows PC, open Command Prompt as Administrator, run
chkdsk D: /f. This clears the dirty flag. Bring it back to your Mac and it should mount. - Option B — macFUSE for read/write access: Install macFUSE plus NTFS-3G, then:
sudo ntfs-3g /dev/disk2s1 /Volumes/MyDrive -o local,allow_other
Fix 4: exFAT or FAT32 Corruption
exFAT is fragile. Eject it improperly once — mid-write, laptop lid shut too fast — and the partition table corrupts. This fix uses macOS's built-in fsck_exfat.
# Unmount first (don't eject)
diskutil unmount /dev/disk2s1
# Run the check
sudo fsck_exfat -d /dev/disk2s1
FAT32 is slightly different:
sudo fsck_msdos -f /dev/disk2s1
Remount after repair:
diskutil mount /dev/disk2s1
Fix 5: Linux Filesystems (ext4, btrfs, xfs)
macOS has zero native support for Linux filesystems. If the drive came from a Linux machine, that's your answer.
- For read access, install ext4fuse via Homebrew:
brew install ext4fuse, then mount with:sudo ext4fuse /dev/disk2s1 /Volumes/LinuxDrive -o allow_other - Just need to grab files off it? Plug it into a Linux machine and copy them over. Much simpler.
Fix 6: Hardware and Connection Problems
Drive doesn't appear in diskutil list at all? The issue is physical.
- Try a different port and cable. USB-C cables are notorious for this — many are charge-only, with no data lines. Use the cable that shipped with the drive.
- Add a powered USB hub. A 2.5" HDD draws around 900mA at startup. MacBook USB ports sometimes can't deliver that, especially with other devices connected.
- Test on another Mac or PC to rule out a failing drive.
- Reset SMC on Intel Macs: Shut down, hold Shift+Control+Option+Power for 10 seconds, then boot. On Apple Silicon, a plain restart does the same thing.
Fix 7: Reset USB Drivers Without Rebooting
On Ventura and Sonoma, you can kick the USB subsystem without a full restart:
sudo kextunload -b com.apple.driver.usb.cdc
sudo kextload -b com.apple.driver.usb.cdc
Unplug and reconnect the drive after running both commands.
Last Resort: Recover Data Before You Reformat
Nothing worked and the drive has files you can't lose? Don't format yet. TestDisk is free, open-source, and surprisingly effective at rebuilding corrupted partition tables:
brew install testdisk
sudo testdisk /dev/disk2
Inside TestDisk: go to Analyse → Quick Search. It scans for lost partitions. If it finds yours, hit Write to restore the table. I've seen this recover drives that looked completely gone.
Partition table is beyond repair but files are still there? Run PhotoRec (bundled with testdisk) for file-level recovery:
sudo photorec /dev/disk2
Verify the Fix
Once something mounts, confirm the drive is actually healthy:
# Check it's listed with a mount point
diskutil list
# Confirm the mount
mount | grep disk2
# Verify read and write both work
ls /Volumes/YourDriveName
touch /Volumes/YourDriveName/test.txt && echo "Write OK"
Mounts and passes the write test? You're done. Always eject properly — diskutil eject /dev/disk2 or drag to Trash in Finder — before unplugging.