What Was Happening
A customer from Clapham SW4 brought in a MacBook Air 13-inch 2018 (A1932) that had stopped booting. The machine would power on, show the Apple logo and a progress bar that advanced slowly to about two-thirds, then restart and repeat the cycle indefinitely. The customer had not been able to access the machine for two days.
The fault had begun immediately after the machine had shut down unexpectedly during a macOS update — a power outage had interrupted the update process mid-installation.
The customer had two years of documents and photos on the machine and was primarily concerned about their data.
Our Diagnosis
The boot loop pattern — progress bar advancing part of the way then restarting — is characteristic of a system volume problem rather than hardware failure. In this case, the interrupted update had left the macOS system volume in a partially written state that could not complete the boot process.
We booted the machine from an external macOS recovery drive to access it without relying on the internal system. From Recovery Mode we ran Disk Utility’s First Aid function on the internal volume. Disk Utility reported errors on the system volume and initiated repairs — the volume had several file system inconsistencies from the interrupted write.
While in Recovery Mode we also ran Apple Diagnostics to confirm the hardware was healthy. Memory, SSD, and logic board all passed. This was a software/file system fault, not a hardware failure.
How We Fixed It
Disk Utility First Aid resolved some but not all of the file system errors. The system volume was still not in a state the macOS installer would accept as bootable. The cleanest resolution at this point was a macOS reinstallation via Recovery Mode, which writes a fresh system volume while leaving the user data partition intact.
We confirmed the user data was accessible before starting the reinstall — mounted the data volume and verified the Documents, Desktop, and Photos folders were all present and intact. The customer’s data was not at risk.
The macOS reinstall ran successfully, overwriting the corrupted system volume and leaving all user data in place. On reboot, the machine loaded to the login screen normally.
Post-reinstall checks: all applications were accessible (applications installed from the App Store reappear automatically after reinstall; third-party applications installed from the web were intact), all documents and photos were present, and system preferences were restored.
The Result
The MacBook Air was returned to the Clapham customer the same day. The machine was booting normally. All two years of documents and photos were intact. The customer did not need to restore from a backup.
Why Update Interruptions Cause Boot Failures
macOS updates write new system files to the system volume while the machine is running. The process involves a staged installation that completes partly during the update download and partly during a restart phase. If the machine loses power during the restart phase — when the final system volume writes are being committed — the result is a volume that contains a mix of old and new files that the boot loader cannot reconcile.
Apple introduced the APFS volume format and snapshot-based updates in later macOS versions partly to make this scenario more recoverable. The A1932 runs macOS Catalina and later, which use snapshot-based updates — but a power cut mid-write can still corrupt the volume beyond what the snapshot can recover, particularly if the outage occurred during the snapshot commit itself.
Prevention Tips
- Never force-shut down or close the lid during a macOS update — allow the full restart and installation to complete before using the machine
- Ensure the laptop is connected to power before starting a macOS update — do not update on battery in a flat where power outages are possible
- Back up with Time Machine before any major macOS update; in this case the data was intact, but that is not guaranteed in all update interruption scenarios
- If the machine has not updated macOS in more than a year, check current storage availability before updating — insufficient free space can cause update failures that produce similar symptoms
Local Help in Clapham SW4
We carry out MacBook startup fault diagnosis and repair at our Putney workshop for customers across Clapham SW4 and SW11. Free collection is available — parking around Clapham Junction is difficult and collection avoids the hassle.
Related Services
- MacBook Repair — software and hardware repairs for all MacBook models
- MacBook Repair in Clapham — local service for SW4 and SW11
More Case Studies
- Faster, safer MacBook Air upgrade to latest macOS — software service on a MacBook Air from a similar generation
- MacBook Pro battery swelling repair in Putney — hardware fault requiring internal component replacement
- MacBook Air hard drive replacement in Streatham — storage fault causing similar slowness and boot issues