macOS updates are essential for maintaining performance, security, and access to the latest features. However, sometimes users encounter unforeseen issues that obstruct smooth installation. A common and particularly stubborn one involves the Preboot volume — an invisible but integral part of APFS (Apple File System) introduced with macOS High Sierra. When this Preboot volume becomes corrupted or misaligned, standard update processes may come to a screeching halt.
TL;DR
If your macOS update refuses to install and stalls during “Preparing Update,” the problem might stem from a problematic or unmounted Preboot volume. This issue is often fixable without reinstalling macOS or erasing your drive. The solution lies in Disk Utility’s command-line sibling, which allows you to manually mount and repair the Preboot volume. A few Terminal commands executed from macOS Recovery can resolve the issue and get updates flowing again.
Understanding the Preboot Volume
The Preboot volume is part of the APFS container structure created when a Mac is formatted using the Apple File System. It contains boot-related data, including necessary files for macOS startup and Secure Boot processes on T2 and Apple Silicon Macs. The Preboot volume is generally invisible to users but crucial to initiating system recovery, updates, and restarts properly.
Problems arise when this volume becomes corrupted, deleted, or simply refuses to mount. This can happen after a forced shutdown, a failed update, or improper disk maintenance with third-party tools. When macOS updates are pushed — especially big ones like point releases or security updates — the installer attempts to write certain boot files within the Preboot structure. If that step fails, the update crashes or gets stuck indefinitely.
Symptoms of the Issue
Identifying a stuck update caused by Preboot problems isn’t simple at first glance, but there are some tell-tale signs:
- macOS update stalls at “Preparing Update…” with no progress
- System restarts multiple times but never completes update
- Diagnostics in Recovery mode or with verbose boot show Preboot volume missing or unmounted
- Software Update says the update is installed — but About This Mac reflects the old version
Apple’s standard advice often leads to reinstalling macOS, which may not fix the Preboot corruption — leading users into a loop of reinstall attempts or forced erases. Fortunately, there is a more direct method to repair the problem using Terminal in Recovery.
The Disk Utility Terminal Trick That Can Fix It
Disk Utility, accessible in macOS Recovery mode, lets users inspect and manage volumes graphically. However, using Disk Utility from the Terminal opens up more precise control. This is especially helpful when volumes are invisible or improperly mounted in the GUI. In this case, we can leverage Terminal to mount the Preboot volume and synchronize its contents with the System volume.
Follow these steps cautiously and ensure you have a full backup using Time Machine or another method before proceeding:
Step 1: Boot into macOS Recovery
- Turn off your Mac completely.
- Turn it on and immediately press and hold:
- Intel Mac: Command + R
- Apple Silicon Mac: Power button until options appear, then select Options > Continue
- Wait until macOS Recovery loads.
Step 2: Open Terminal
In the macOS Utilities window, click Utilities > Terminal.
Step 3: Identify Your System Disk
Run the following command to list APFS containers:
diskutil listLook for something labeled “Macintosh HD” or your custom system volume name. Note the disk identifier, such as disk1s5. Also look for “Preboot” — often something like disk1s3.
Step 4: Mount the Preboot Volume
Now, mount the Preboot volume manually:
diskutil mount disk1s3Replace disk1s3 with your actual Preboot volume identifier. You should see a confirmation like “Volume Preboot on disk1s3 mounted.”
Step 5: Create the Correct Folder Structure
Once mounted, navigate inside the Preboot volume and find the UUID (a long alphanumeric identifier) that corresponds to your main system volume. To check:
ls /Volumes/PrebootYou should see a folder resembling 1A2B3C4D-1234-5678-ABCD-1234567890AB. If not, or if the folder appears empty or corrupted, you can regenerate it using:
bless --info /Volumes/Macintosh\ HDThen sync the Preboot volume:
bless --folder "/Volumes/Macintosh HD/System/Library/CoreServices" --bootefi --create-snapshotStep 6: Reattempt the Update
After performing these actions, reboot your system normally and attempt the update again via Software Update. In most cases, the update completes without further interruption.
Why This Trick Works
The bless command is underutilized but immensely powerful. It directly configures how a Mac boots, including setting system integrity snapshots for APFS boot volumes. By syncing the Preboot structure to match the latest system data, you’re ensuring both the update utility and startup sequence are speaking the same language again.
[p ai-img]mac updating software update system preferences[/ai-img]
This method avoids the need for a full reinstall or Disk Utility erase, saving time and preserving user data.
Precautions and Additional Tips
- Always back up your system — Time Machine or a bootable clone are preferred.
- Use Terminal only if you’re comfortable interpreting disk structures and following detailed steps.
- If bless fails repeatedly, try first rebooting into Safe Mode and then Recovery again — caching issues sometimes interfere with Preboot updates.
- Be meticulous with disk identifiers — using the wrong disk could harm data.
Apple may eventually fix this type of bug with stronger Preboot resilience in future macOS versions. But until then, this Terminal-rescue method remains one of the most reliable ways to resolve it.
Conclusion
When macOS updates refuse to install due to a corrupted or unmounted Preboot volume, the root cause is more than a cosmetic hiccup — it’s a structural inconsistency deep in the disk’s partition map. Fortunately, Disk Utility’s Terminal interface and the bless tool provide direct ways to repair and reuse the very parts of your system that macOS relies on to stay updated.
Understanding how macOS structures its startup and update systems can empower users to solve what at first feels like a catastrophic failure. By treating the Preboot volume with the attention it deserves, and repairing it using the method described above, many users can save hours and protect their data — all without having to reinstall the OS.
