External hard drive won't mount on Apple Silicon Mac

After this, the drive shows up in Finder and on the Desktop the moment it's plugged in, same as any other Mac.

Why this happens

It's worth debunking a specific myth first: Apple's own platform security documentation describes Apple Silicon's Secure Boot policy (Full Security vs. Reduced Security) as governing which macOS versions and kernel extensions are allowed to boot, and it only comes into play at all when you're trying to start the Mac up from external media — it has nothing to do with reading or writing a normal external data drive during regular use. If a drive won't mount, the boot-security system isn't why.

The two real, common causes are more mundane. First, file-system trouble: exFAT and NTFS drives are more prone to directory corruption when unplugged without ejecting properly, and macOS's write support for NTFS is deliberately limited. Second, power delivery: a bus-powered external drive plugged into a bus-powered USB-C hub, alongside other peripherals, can exceed what the hub or the Mac's port budgets for that connection — macOS has been reported to disconnect a device outright rather than let it brown out mid-write. Plugging the same drive directly into the Mac, or into a powered hub, resolves this class of issue immediately.

Fix it

1. Try the interface path first: open Disk Utility, select the drive (not just the volume beneath it), and click First Aid. This repairs common directory-structure problems without touching your files, and is the normal, intended first step — reach for Terminal only if this doesn't resolve it.

2. Confirm what macOS actually sees at the device level:

   ● Reads the system

   What it does: lists every disk and partition macOS currently recognizes, mounted or not.

   After: nothing changes, this only reads.

   diskutil list

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   If your drive doesn't appear at all, this points at a cable, port, or power problem rather than a file-system one — try a different cable and a direct connection to the Mac before going further. If it appears but isn't mounted, continue to the next step.

3. If Disk Utility's First Aid didn't fix it, or you want to see more detail than the GUI shows, verify and repair from Terminal using the disk identifier from the previous step (for example disk4, not a specific slice like disk4s1):

   ▲ Changes settings

   What it does: checks and repairs the disk's directory structure.

   After: if repairable, the drive should mount normally afterward; this does not erase files.

   sudo diskutil repairDisk /dev/disk4

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   Revert: not applicable — this repairs structure, it doesn't change anything you'd want to undo.

   If repair fails outright rather than fixing anything, that's a sign of real hardware failure or corruption beyond a structural fix — back up whatever's still readable before doing anything else with the drive.

Still broken?

• Your internal disk is the one showing storage problems, not an external drive
• The internal disk reads full and you can't find why

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Related problems

• Disk full but no large files to delete
• System Data category is huge on Mac