Running First Aid or fsck on failing drives can worsen corruption. Back up if the disk mounts even once.
What you will achieve
Get an external USB, Thunderbolt, or SD card mounting again on macOS.
External disks fail to mount from cable faults, filesystem corruption, incompatible formats, or insufficient power. Disk Utility shows greyed volumes invisible in Finder. Distinguish read-only NTFS from true mount failure before erasing a disk that still holds data.
1) Physical layer
- Try another port, cable, and powered hub if drive needs extra power.
- Listen for click of death on HDDs — mechanical failure needs lab recovery.
- Test drive on another Mac or PC to isolate host vs disk.
2) Disk Utility
- Open Disk Utility — show all devices (View → Show All Devices).
- If disk greyed out, select container → Mount.
- Run First Aid on container and parent disk.
3) APFS encryption
Encrypted volumes need password in Disk Utility → Mount. Forgotten password = data loss without backup.
4) fsck in Recovery
If First Aid fails, boot Recovery → Terminal:
diskutil listNote identifier; run First Aid from Disk Utility GUI on unmounted disk before destructive steps.
5) Avoid format trap
Erasing fixes mount but deletes data. Only erase after recovery attempts or if disk is confirmed empty.
6) NTFS and exFAT read-only
macOS mounts NTFS read-only by default — not “failed mount” but no write. exFAT corruption shows as unmounted — run First Aid; if fails, copy what mounts on Windows PC then reformat.
7) Power management
Bus-powered HDDs brown out on USB-A ports — use powered hub or Y-cable if vendor supplied. In Disk Utility, show log for I/O errors indicating cable.
8) Target Disk Mode
Intel Macs: boot holding T to mount internal disk on another Mac for data salvage. Apple Silicon: Share Disk in Recovery achieves similar — use for pulling data before erase.
Verify
Volume mounts on desktop; files readable; First Aid reports OK or repaired.
Additional troubleshooting notes
If steps above do not resolve the issue on the first attempt, reboot once, confirm System Settings → General → Software Update is current, and retry with a second administrator account to rule out profile or keychain corruption in your daily user. Document exact error text from Console.app with timestamp — vague “it still fails” without logs wastes support time. On Apple Silicon, re-test after full shutdown (not just restart) because firmware and Thunderbolt controllers reset only on cold boot. Intel Macs should repeat test in Safe Mode to bypass third-party login items. Before erase or keychain reset, verify Time Machine or clone backup completed — batch 3 guides assume Monterey/Ventura/Sonoma/Sequoia paths in System Settings; search Spotlight for renamed panes if your macOS version labels differ slightly.