At some point, many macOS users encounter the same unsettling moment: storage is nearly full, and the majority of the disk appears to be consumed by something called System Data. In my case, that number exceeded 130 GB. There were no unusually large documents, no massive downloads, and no obvious culprit.
This post documents the full journey I took to understand what that number really meant, how macOS classifies storage, and how I safely reclaimed a very large amount of disk space without breaking the system or losing personal data.
I am writing this as a computer scientist, but intentionally in a calm and approachable tone. The goal is not to rush or apply hacks, but to understand what is happening and act deliberately.
Defining the Problem
macOS storage categories are broad by design. System Data is not a single thing. It is a bucket that includes caches, internal databases, sandboxed application data, and analysis artifacts. Importantly, it often includes files that live inside your user account, even though they are labeled as system owned.
The symptoms were straightforward:
- Available disk space was critically low
- System Data alone accounted for roughly 134 GB
- User-facing folders such as Documents and Downloads were relatively small
The real danger at this stage is panic. Random deletion inside Library or System folders can easily cause permanent damage. The priority was correctness, not speed.
Stop Guessing and Measure First
The first rule I followed was simple: never delete what you have not measured.
Rather than relying solely on the macOS Storage interface, I inspected disk usage directly. This immediately revealed an important fact. The operating system itself was not the primary consumer of space.
The majority of the disk usage lived inside my home directory, specifically:
~/Library/Containers
This folder alone accounted for more than 90 GB. At that point, the problem stopped being mysterious. The space was user-level data that macOS was categorizing imprecisely.
What Containers Really Are
Containers are sandboxed storage areas used by modern macOS applications. They hold caches, indexes, temporary processing data, and derived assets. These files are often safe to regenerate, but they are not automatically cleaned up.
A closer look showed three dominant contributors:
- Photos video conversion caches
- Photos media analysis data
- Docker application data
This write-up focuses on the Photos-related components, which were both the largest and the least obvious.
The Photos Analysis Accumulation
Photos performs extensive background work: face recognition, object detection, video transcoding, and content analysis. All of this is legitimate, but it produces a large amount of derived data.
Two container folders were responsible for the majority of the space:
com.apple.photos.VideoConversionServicecom.apple.mediaanalysisd
Together, these folders consumed well over 70 GB. None of this data was original photos or videos. It was generated output that macOS can rebuild when necessary.
The Critical Rule: Stop the Processes First
One important lesson is that macOS will immediately regenerate these caches if the related background services are running. Deleting files while the system is actively using them is ineffective.
The correct sequence was:
- Quit Photos completely
- Ensure photo and media analysis processes were stopped
- Delete only the specific container folders identified earlier
- Restart the system and allow it to settle
This is not a workaround or exploit. It is controlled cache invalidation.
A Note on Temporary Folders
During the cleanup, macOS briefly exposed a temporary directory that appeared to contain familiar folder names such as Documents and Pictures. This can be alarming if you encounter it unexpectedly.
These were aliases, not real data. Temporary workspaces often mirror structure without owning content. Nothing personal was deleted, and this behavior is expected during large cache cleanup operations.
The Outcome
After restarting and allowing macOS to recalculate storage usage, the results were clear:
- System Data dropped by more than 50 GB
- Disk pressure was eliminated
- No personal data was lost
- The system remained stable
Photos continued to function normally. Background analysis resumed gradually rather than all at once, which is exactly the desired behavior.
Final Thoughts
The key takeaway is that System Data is not untouchable or mysterious. It is often poorly labeled user-level storage.
The second takeaway is discipline. Measure first. Identify the largest contributors. Stop relevant services. Delete only data that is clearly derived and rebuildable.
If you approach the problem this way, you can safely reclaim tens or even hundreds of gigabytes without third-party cleaning tools or risky system modifications.
macOS is conservative by design. If something is truly required, it will return on its own. That alone is a strong signal that responsible cleanup is not only possible, but expected.