How to Fix the mount:/ failed with 77 Error Mac?

TL;DR:

The "mount:/ failed with 77" error means macOS can't mount a disk (internal or external) — usually a bad USB connection, disk errors, an unsupported file system, or insufficient power. Fix it in order: 1) reseat the USB cable/port or use a powered hub; 2) run diskutil list then diskutil mountDisk /dev/diskX in Terminal; 3) open Disk Utility > First Aid to repair without erasing; 4) only as a last resort, Erase the disk to a macOS-compatible format — after recovering data with Wondershare Recoverit if the disk holds important files.

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"mount:/ failed with 77" means your Mac can't mount a disk — internal or external — so you can't read or write to it. Most cases are fixable at home: bad connection, disk errors, unsupported format, or power issues. This guide covers the fixes and how to recover data if the disk still won't mount.

Part 1: What is the Meaning of Mount Failed with 77?

The "mount:/ failed with 77" error appears when macOS can't mount a disk — internal drives and USB external drives alike — leaving the data inaccessible. Understanding the cause (connection, disk health, file system, power) lets you fix it faster and avoid unnecessary data loss.

Part 2: What Causes the Mac Mount Failed with 72/77 Error Mac?

There are several reasons why you might see the "Mount Failed with 77" or "Mount Failed with 72" error on your Mac:

Part 3: Fixing the "Mount:/ Failed with 77" Error Mac: Easy Fixes

Work through these fixes in order — most mount failures are solved by the first two:

Fix 1: Check the USB Port/Cable

Sometimes, the reason your Mac shows the "mount:/ failed with 77" error is simply a bad connection. Start by checking if your USB cable and port are in good condition. Try using a different USB port on your Mac or use another cable. This can often solve the problem if the issue is just a poor connection. Once you've checked and possibly changed the port or cable, re-insert the disk.

If it mounts correctly, then you know the problem was with the connection. This is an easy and quick fix that you can try before moving on to more complicated solutions.

Fix 2: Use the Diskutil Command

If checking the connection doesn't resolve the issue, you can attempt to force mount the disk using Terminal. Terminal is a robust tool included with your Mac that lets you input commands to manage your computer.

1. First, open Terminal. Then, type diskutil list. With the help of this command, you will see all the disks connected to your Mac. Find the disk you want to mount and note its number (like /dev/disk2).

   

2. Next, type diskutil mountDisk /dev/diskX (replace "X" with your disk number) and press Enter. This command tells your Mac to mount the disk, and it can help if the disk wasn't mounting automatically.

If the disk mounts but shows errors, run diskutil verifyVolume /dev/diskXs1 (replace X and s1 with your disk and volume numbers) to check the file system without changing it.

Fix 3: Repair or Erase the Mac Disk

If the disk still won't mount, try Disk Utility > First Aid first — it verifies and repairs the file system without erasing data. If First Aid reports an unsupported or damaged format and the drive is empty (or data is backed up), you can erase it to a macOS-compatible file system. Erasing destroys all data — recover important files with a tool like Wondershare Recoverit first if needed.

1. Open Disk Utility, select the disk, and click First Aid to attempt a non-destructive repair. If that fails, back up any files you can still access, then select the disk and click Erase.

   

2. Click Erase, choose a macOS-compatible format like APFS or Mac OS Extended (Journaled), and then click Erase again to start the process.

Once the disk is reformatted, it should mount correctly, and you can use it without any issues.

These steps should help you fix the "mount:/ failed with 77" error on your Mac, allowing you to access your drives and files again. If the problem persists, it might be worth seeking professional help to ensure there's no deeper issue.

Part 4: Lost Data Due to Mount Failed with 77 Error? Use Recoverit Today

Wondershare Recoverit recovers files from an unmounted external drive on Mac — whether the data was lost to the mount error, accidental deletion, or corruption.

Wondershare Recoverit

• Recovers data from 1000+ file formats across HDDs, SSDs, USB drives, SD cards and more.
• Handles 10000+ data loss scenarios, including deletion, formatting, corruption, and virus attacks.
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Steps to Use Recoverit:

1. Get Recoverit data recovery software and install it on your Mac. Open Recoverit and choose the drive where the data was lost. Click "Start".

   

2. Wait for the scanning process.

   

3. Preview the found files and select the ones you want to recover. Click "Recover" to save them back to your Mac.

   

Video Tutorial on How to Recover Lost Files After a macOS Update

Part 5: Tips To Avoid The Mac Disk Unmounted Issues

Prevent unmount failures with these habits:

Conclusion

Fix the "mount:/ failed with 77" error in order: reseat the cable, force-mount with diskutil, run First Aid to repair without erasing, and only then erase the disk. If important files are trapped on the unmounted drive, recover them with Wondershare Recoverit before reformatting — then back up regularly to avoid the problem next time.