It can be frustrating when you connect an external hard drive to your Mac, only to have it randomly eject or unmount without warning. There are several potential causes for this issue that are worth investigating.
Quick Answers
– Faulty cable connection between the drive and computer
– Insufficient power supply to the external drive
– Incompatible file system format on the drive
– Drive errors and corruption
– Outdated software/drivers on the Mac
– Competing processes trying to access or eject the drive
Checking Cable Connections
One of the simplest explanations for a Mac frequently ejecting an external drive is a faulty cable connection. Here are some tips for troubleshooting the physical connections:
- Inspect both ends of the USB, Thunderbolt, or FireWire cable for any damage or bent pins.
- Try connecting the drive to another port on the Mac if available.
- Test the drive and cable with a different computer if possible.
- Replace the cable if you suspect it is damaged.
- For desktop drives, check that the power cable is securely connected.
- Make sure the drive is properly seated in the enclosure if it is detachable.
A partially loose cable can cause intermittent signal drops that may look like random external drive ejects on a Mac. Reseating connections or swapping out a faulty cable is often an easy fix.
Power Supply Issues
External hard drives and SSDs require power to operate, whether they are powered over the connecting interface cable or by a separate power cord. If the drive is not receiving consistent power, it may unexpectedly disconnect from the Mac.
For drives with a dedicated power adapter, make sure it is plugged into a working outlet. Try using a different power cable if you have one available. Utilizing a higher capacity power adapter can also resolve underpowering issues in some cases.
Bus-powered devices should work properly with the standard power provided by USB/Thunderbolt ports on a Mac. But you may need to use an externally powered USB hub if the drive is trying to draw too much power from the port.
File System Incompatibility
The external drive must be formatted with a file system that macOS understands in order to mount and stay mounted. If the drive is formatted with an incompatible file system, the Mac will fail to recognize it and auto-eject it.
Most external drives come preformatted with the FAT32 or exFAT file systems for maximum compatibility. macOS can read and write to these formats without issue.
However, if the drive was erased and formatted with a Linux file system like EXT4 or XFS for example, it would not properly mount on a Mac. The drive needs to be reformatted with a macOS compatible file system to resolve this ejection problem.
Common File Systems for Mac External Drives
| APFS | Native macOS file system since High Sierra |
| HFS+ | Legacy Mac format, read/write on modern macOS |
| ExFAT | Cross-platform, max file size >4GB |
| FAT32 | Cross-platform, max file size 4GB |
Drive Errors
If your external drive is ejecting/unmounting itself due to underlying errors, that indicates a problem with the drive itself. Some potential errors that can cause this behavior include:
- Bad sectors – Damaged portions of the drive platters
- Clustering failures – Invalid data clusters on the drive
- Corrupted system files – Critical file system structures are damaged
- Drive motor or head issues – Mechanical failure
Most external hard drives use either spinning platters or flash memory to store data. Physical problems with either the platters or flash memory can cause read/write errors that interfere with proper drive operation and lead to spontaneous ejection on Macs.
Checking Disk Health
To check an external drive for errors, you can use macOS built-in Disk Utility. Select the disk name from the left sidebar, then go to First Aid tab and click “Run”. This will scan the drive and attempt any repairs.
If the First Aid scan shows multiple errors or is unable to repair issues, it likely indicates a drive hardware problem or failure. At that point it may be necessary to replace the external drive.
Outdated Drivers, Firmware, or macOS
Software and driver issues can also cause random ejection problems with external drives:
- Old USB/Thunderbolt drivers – Update Mac to latest version of macOS
- External drive firmware – Check manufacturer website for updates
- USB/hub controller issues – Try disconnecting hub if applicable
- Incompatible system software – Reset NVRAM and SMC if needed
Keep your Mac and connected devices updated to avoid software conflicts. Restarting both the Mac and external drive may also help clear any stuck processes or faults.
Competing Processes
If there are multiple programs trying to access the external drive at the same time, they can end up competing for resources and inadvertently cause the drive to disconnect. Some examples include:
- Antivirus or backup software trying to scan files
- Spotlight indexing the drive in the background
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- Time Machine trying to backup external drive files
- Downloading/moving large files to the drive
If you notice the ejections happening during a particular activity, try disabling that process temporarily to see if it resolves the frequent ejection behavior.
Corrupted Preference Files
Damaged or outdated system preference files can cause problems with external drives mounting properly. To reset preferences that may be affecting drive connections, follow these steps:
- Restart your Mac and immediately press Command + R keys to boot into Recovery mode.
- When the Recovery window appears, select Disk Utility > Continue.
- In Disk Utility, choose your Mac’s main internal drive from the sidebar.
- Click the First Aid button and run checks/repairs as needed.
- After scans complete, quit Disk Utility and return to the Recovery window.
- Select Terminal from the Utilities menu.
- In Terminal, type “resetpassword” and press Return.
- When the Reset Password window appears, select your Mac HD and click the Reset button.
- Restart your Mac normally after resetting PRAM. Test the external drive again.
Resetting NVRAM and SMC can also help clear stored system settings and drivers that may be causing conflicts with external drives.
Conclusion
An external hard drive or SSD unexpectedly ejecting/unmounting on a Mac can stem from different technical causes. Start troubleshooting by verifying the physical connections are secure and that power is adequate. Incompatible formatting, drive errors, software conflicts, and corrupted preference files could also contribute to frequent improper ejections.
Test the drive with multiple cables, ports, and computers if possible to isolate the fault. Update macOS, firmware, and drivers as a maintenance step. Scan for and repair drive errors using Disk Utility if present. As a last resort, re-formatting the external drive may be required to fully resolve persistent ejection issues.
