External Hard Drives Not Mounting After macOS Sequoia Update? [Fix]

Since the release of macOS Sequoia (version 15), users across Apple support forums, Reddit threads, and tech communities have reported serious issues with external hard drives and flash storage not mounting properly. In many cases, devices that were working flawlessly before the update are now either invisible, eject randomly, or display errors when being ejected manually.

This post summarizes what has been observed, what has been attempted, and what has been found to work in certain situations.

What Was Reported by Users

Drives Suddenly Not Detected
External drives — including SSDs and flash memory — have failed to appear in Finder or Disk Utility after the Sequoia update. Even brand-new drives were affected, despite functioning well on other systems.
“Disk in Use” Errors When Ejecting
Flash drives have frequently triggered an alert stating they could not be ejected because they were “in use by another program”, even when no apps were open.
Severely Reduced Write Speeds
SSDs that normally wrote at over 300 MB/s were limited to speeds as low as 8 MB/s. Performance degraded unpredictably.
Thunderbolt and USB Ports Unresponsive
For some, no ports — including those used for backups, USB accessories, or external keyboards — functioned at all after the update.
Issues Confirmed by Apple Support
After long sessions with senior Apple technicians, it was acknowledged that the issue stems from macOS Sequoia itself, not from user settings or faulty devices.

What Apple Support Recommended

According to one user who worked with a senior technician:

The bug had been logged internally.
Fixes were being prioritized based on volume of complaints.
Version 15.1.1 of macOS would not fix the issue.
Users were advised not to buy new drives or try third-party tools.
A future update, likely focused on Time Machine and drive access, was expected to resolve the problem.

Until then, patience was recommended.

How to Fix External Hard Drives Not Mounting After macOS Sequoia Update?

1. Update to macOS Sequoia 15.3.1

For many, problems were resolved after updating to version 15.3.1.

2. Reset the SMC (System Management Controller)

This helped restore visibility to external drives.

Apple Silicon Macs:

Shut down the Mac, wait 30 seconds, and restart. The SMC resets automatically.

Intel Macs:

Shut down, then hold Shift + Control + Option + Power for 10 seconds.
Release and restart.

3. Reset NVRAM

This cleared low-level settings affecting drive access.

Shut down the Mac.
On startup, hold Option + Command + P + R for 20 seconds.

4. Reformatting Drives to ExFAT

Some users with Samsung or Crucial SSDs found that formatting their drives to ExFAT restored write speeds and stopped random ejection — especially on M3 and M4 Macs.

Additional Observations

Drives that failed on an M3 Max MacBook worked fine on an older Intel Mac.
Random disconnections occurred while using Logic Pro, causing crashes and data loss.
Some drives were only recognized after multiple reboots or when connected via a USB hub.

Final Thoughts

The macOS Sequoia update introduced changes that negatively impacted external drive mounting, speed, and stability — especially on M-series Macs. Although the problem appears to be software-based, only partial fixes have been seen so far through resets and minor updates. The issue has been acknowledged by Apple internally, but no official patch note has listed it as resolved yet.

Until a full solution is released, users are advised to:

Update to macOS 15.3.1 or later.
Reset SMC and NVRAM.
Avoid reformatting unless necessary.
Monitor future macOS updates that mention Time Machine or external drive fixes.