macOS Sonoma won't mount external drive – 3 fixes

First thing: check Disk Utility

This is the fix that works for about 70% of people. macOS Sonoma (14.x) has a bug where drives that mounted fine in Ventura suddenly stop appearing on the desktop or in Finder. Don't reboot yet. Don't unplug the drive. Just open Disk Utility.

1. Open Applications > Utilities > Disk Utility. Or hit Cmd+Space, type "Disk Utility", hit Enter.
2. Look in the left sidebar under External. If you see your drive listed but it's grayed out or says "Not Mounted", click it once to select it.
3. Click the Mount button in the toolbar (it looks like a triangle pointing right, sitting on a rectangle).
4. After clicking Mount, you should see the drive appear on your desktop and in Finder within 3 seconds.
5. If the Mount button is grayed out, click First Aid (just above Mount). Disk Utility will scan and repair the drive. Wait for it to finish. Then try Mount again.

What you should expect: After First Aid runs, you'll see a green checkmark and "Operation successful." Then the Mount button becomes clickable. After mounting, the drive icon shows up on the desktop. If that happens, you're done. If not, move to the next fix.

I've seen this fail on drives formatted as exFAT or NTFS when the drive was improperly ejected before. First Aid usually catches that. But if you get "The disk couldn't be mounted" with error code 49221, that's a volume header problem – go straight to the Terminal fix below.

Second fix: mount from Terminal

When Disk Utility won't mount the drive or shows it as "uninitialized," the real problem is often a corrupted volume header or a minor filesystem glitch. Terminal gives you more control. Here's what to do.

1. Open Terminal (Applications > Utilities > Terminal).
2. Type diskutil list external and press Enter. You'll see a list of all connected external drives. Look for the one that's not mounting – it'll be something like /dev/disk4 or /dev/disk6.
3. Find the specific partition that's not mounting. It'll have a size that matches your drive. The identifier will look like disk4s1 or disk6s2. Copy that identifier (the s1 or s2 part).
4. Now type diskutil mount /dev/disk4s1 (replace disk4s1 with your identifier) and press Enter.
5. Expected outcome: You'll see "Volume (name) on (identifier) mounted successfully." The drive should pop up on your desktop within 2 seconds.
6. If you get "The volume is already mounted," then the drive is mounted but not showing in Finder. That's a Finder refresh issue. In Terminal, type killall Finder and press Enter. Finder will relaunch, and the drive should appear.
7. If you get "The volume couldn't be mounted" with error -69877 or -69888, that's a corrupt partition map. That's when you need sudo fsck_exfat -d /dev/disk4s1 (for exFAT) or sudo fsck_hfs -fy /dev/disk4s1 (for HFS+). Run that, let it finish, then try the mount command again.

I prefer the Terminal route for stubborn drives because it gives you real error codes. Disk Utility hides them behind "Operation failed." That's not helpful. Terminal tells you exactly what's broken.

Third fix: check for third-party driver conflicts

This one catches people who use drives formatted as NTFS or exFAT with third-party drivers. macOS Sonoma changed how it handles kernel extensions (kexts). Paragon NTFS, Tuxera, and older versions of Microsoft NTFS for Mac can break with Sonoma.

1. Go to System Settings > General > Login Items > Allow in the Background. Scroll the list. If you see anything from Paragon, Tuxera, or Seagate (they use Paragon), that's your suspect.
2. If you see it, toggle it OFF. You don't need to uninstall it yet. Just disable the background service.
3. Reboot your Mac. After the reboot, plug the drive in again. It should mount without the third-party driver interfering.
4. If it mounts, you've got a conflict. You have two choices: update the driver to a Sonoma-compatible version, or use Apple's built-in NTFS support (it's read-only, but you can enable write support via a Terminal command).
5. To enable Apple's NTFS write support (advanced): sudo nano /etc/fstab, then add UUID=YOUR_DRIVE_UUID none ntfs rw,auto,nobrowse. Find the UUID by running diskutil info /dev/disk4s1 | grep "Volume UUID".

Expected outcome: After disabling the third-party driver and rebooting, the drive mounts normally. If it still doesn't mount, the problem isn't the driver. Go back to the Terminal fix.

I've seen this happen most often with Seagate Backup Plus drives that come with Seagate's Dashboard software. That software installs a Paragon-based NTFS driver that hasn't been updated for Sonoma 14.3 or later. Uninstalling the Dashboard entirely and using Apple's NTFS support fixed it every time.

Quick-reference summary

One last thing: if none of these work, the drive itself might be dead. Plug it into another Mac or a Windows machine. If it doesn't show there either, it's hardware failure. Time for a new drive. But if it shows on another machine, the problem is specific to your Sonoma setup – try a PRAM reset (reboot, hold Cmd+Option+P+R until you hear the chime twice). That clears NVRAM settings that can interfere with USB detection.