First Aid repairs filesystem metadata — it does not recover failing hardware. If the drive clicks, smells, or First Aid reports unrepairable errors, stop running it repeatedly and clone data immediately or contact recovery specialists.
What you will achieve
Run Disk Utility First Aid on an external USB or Thunderbolt drive to fix minor APFS or HFS+ filesystem errors — unmount issues, directory corruption, “cannot be repaired” warnings in Finder — before escalating to data recovery tools.
1) When First Aid helps
- Drive mounts slowly or ejects unexpectedly.
- Finder reports “The disk is not readable” but the drive appears in Disk Utility.
- After unsafe ejection or power loss during write.
- Before erasing — sometimes repair avoids data loss.
2) Open Disk Utility and show all devices
- Connect the external drive directly — avoid unpowered hubs for failing disks.
- Open Applications → Utilities → Disk Utility.
- Choose View → Show All Devices.
- Select the physical disk (top level), not only the volume beneath.
3) Run First Aid on the volume
- Select the APFS volume or partition (e.g. “Backup” under the physical disk).
- Click First Aid → Run.
- Wait for completion — large spinning disks take time; do not disconnect.
If volume-level First Aid fails, run First Aid on the container, then the physical disk top level.
4) APFS vs HFS+ notes
Modern externals use APFS (macOS Big Sur+ default). Older drives may be Mac OS Extended (HFS+). First Aid handles both; cross-platform ExFAT volumes use different repair paths — Windows chkdsk on a PC if ExFAT is corrupt. Apple Silicon Macs format new externals as APFS by default; Intel Macs may still have HFS+ Time Machine disks from years ago.
5) When the disk will not mount
Grayed-out volume in Disk Utility: run First Aid on the physical disk. If “Operation failed” with hardware error codes (I/O error), assume hardware failure. One successful First Aid after soft corruption is fine; repeated failures mean stop.
6) First Aid from Recovery
If the internal boot disk needs repair, boot Recovery (Apple Silicon: hold power → Options; Intel: Command+R) and run Disk Utility there. For externals, normal boot is usually sufficient unless the external is your only boot drive.
7) After successful repair
Eject properly, reconnect, verify files open. Copy critical data to another disk if the drive showed SMART warnings or repeated errors — repaired filesystems on failing hardware buy time, not permanence.
8) Intel vs Apple Silicon external boot
Apple Silicon cannot boot macOS from all external volumes without explicit bless — First Aid on externals used for data (not boot) behaves the same on both architectures. T2/Apple Silicon Macs may encrypt externals with hardware — note password before repair attempts.
Verify
Disk Utility reports green success or “appears to be OK”; volume mounts in Finder; sample files read without I/O errors.