When you encounter the "The mounted backup volume is inaccessible" error, it is usually because macOS cannot access the connected and mounted Time Machine backup drive. 1. First check the connection and disk recognition to ensure that the drive is properly connected, try re-plug or connect directly to the Mac, and check whether it is displayed in Disk Utility. If it is not mounted, it will be mounted manually. 2. Use the Disk Tool to verify and repair the disk, select the backup volume to run the "First Aid" function. If it cannot be repaired automatically, you need to back up the data and reformat the drive. 3. Reset Time Machine preferences, go to /Library/Preferences/ to find the com.apple.TimeMachine.plist file and move it to the desktop, restart and then scan the backup disk again. 4. Reformat the drive to APFS or Mac OS extension format, and after backing up your personal files, formatting operations to ensure compatibility. The above steps usually solve this problem.
If you're seeing the error "The mounted backup volume is inaccessible," it usually means macOS can't access a Time Machine backup drive that's connected and mounted. This can happen for various reasons—like permission issues, disk errors, or even how the drive was formatted. Let's go over some practical fixes.

1. Check the Connection and Disk Recognition
Before jumping into complex solutions, make sure your backup drive is properly connected and recognized by your Mac.
- Try unplugging and replugging the drive.
- If it's a USB hub, connect directly to your Mac.
- Open Disk Utility and see if the drive shows up in the left sidebar.
- If it doesn't appear at all, try restarting your Mac with the drive plugged in.
- If it appears but isn't mounted, right-click and choose Mount .
Sometimes the system just needs a nudge to recognize the volume properly.

2. Verify and Repair the Disk Using Disk Utility
Even if the drive mounts, there might be file system issues preventing macOS from accessing it as a backup volume.
Here's what to do:

- Open Disk Utility .
- Select the backup volume (not the whole disk).
- Click First Aid , then Run to check and repair disk errors.
If First Aid reports issues that it couldn't fix automatically, you may need to back up any important data (if possible), reformat the drive, and start fresh. Just note: formatting will erase everything on the drive.
3. Reset Time Machine Preferences
Time Machine keeps internal records of drives it trusts. Sometimes those get corrupted or mismatched.
To reset:
- Navigate to
/Library/Preferences/
(you can use Finder > Go > Go to Folder). - Look for a file named
com.apple.TimeMachine.plist
. - Move it to your desktop (don't delete it yet).
- Restart your Mac.
- Reconnect the drive and open Time Machine preferences to see if the volume now appears.
This basically tells macOS to forget previous settings and re-scan for valid backup disks.
4. Reformat the Drive as APFS or Mac OS Extended
Time Machine works best with certain formats. If the drive was reformatted or used on other systems, that could cause incompatibility.
If you're OK with wiping the drive:
- Back up any personal files first (if you can access them).
- Open Disk Utility .
- Select the drive.
- Click Erase .
- Choose either:
- APFS (Encrypted or not) – good for newer Macs with SSDs.
- Mac OS Extended (Journaled) – more compatible with older macOS versions.
Once formatted, reconnect and let Time Machine set it up again as a backup drive.
Fixing this issue often comes down to checking connections, repairing the disk, resetting preferences, or making sure the format is correct. Most of the time, one of these steps should clear things up.
Basically that's it.
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