How I Found 43GB of Hidden Data on My Mac

I was checking the storage on my Mac recently when I noticed that System Data was taking up way more space than I expected.

If you’ve used macOS for a while, you’ve probably seen this category before. The annoying thing is that Apple tells you how much space “System Data” is using, but doesn’t really tell you what that data is.

So I got curious.

I opened Terminal and, with a little help from ChatGPT, started digging through my storage to figure out where all that space was actually going.

We started from the top and worked our way down. My Mac’s Data volume was using about 161GB, and 123GB of that was coming from my user folder.

That narrowed things down considerably.

Then we checked my Library folder.

99GB.

That immediately looked suspicious.

Going down the rabbit hole

The next step was figuring out what inside ~/Library was responsible for those 99GB.

The biggest folders were:

37GB Group Containers 37GB Application Support 13GB Containers 9.7GB Caches

At this point, ChatGPT had me drill into each one rather than just start deleting things — which is important because your Library folder contains plenty of stuff you absolutely shouldn't randomly remove.

And that's when we found it.

WhatsApp was sitting on 30GB of data.

Almost 29GB of that was media.

The funny part?

I don't even have WhatsApp installed on my Mac anymore.

I switched to WhatsApp Web a while ago and deleted the desktop app, so naturally I assumed its data went with it.

Apparently not.

All the photos, videos, documents and other media WhatsApp had downloaded while I was using the desktop app were still sitting inside:

~/Library/Group Containers/

Deleting the application had removed the app, but not all the data it had accumulated.

Then I found another 12GB

WhatsApp wasn't the only abandoned app still occupying space.

I had previously installed UTM to run a Windows 11 virtual machine on my Mac. I hadn't used it in a while and had already uninstalled UTM.

But when we inspected ~/Library/Containers, there it was:

Windows11.utm — 12GB

So once again:

The app was gone.

Its data wasn't.

Between WhatsApp and UTM alone, there was roughly 42GB of data belonging to applications I didn't even have installed anymore.

After confirming what the folders belonged to, I deleted the leftovers.

Then I ran the storage check again.

My Library folder went from:

99GB

to:

56GB

43GB recovered.

Not from deleting applications I use.

Not from moving my photos to an external drive.

Not from some Mac cleaner app.

Just old application data that had been sitting there unnoticed.

The commands that helped me find it

For anyone who wants to investigate their own Mac, this was where we started:

sudo du -xhd 1 /System/Volumes/Data 2>/dev/null | sort -hr | head -30

That shows the biggest directories on your Data volume.

From there, I checked my home directory:

du -xhd 1 ~ 2>/dev/null | sort -hr | head -30

And then my Library:

du -xhd 1 ~/Library 2>/dev/null | sort -hr | head -40

Once we knew Library was the culprit, it was just a matter of continuing to drill into the largest directories until we found what was actually taking up the space.

One thing I wouldn't recommend is seeing a huge Library folder and immediately running rm -rf on things.

Don't do that.

Some of these directories contain application databases, settings, browser profiles, virtual machines and other important data. In my case, I used ChatGPT to help interpret the folder names and sizes before deciding what was actually safe for me to remove.

What I learned

The biggest takeaway for me wasn't really the 43GB.

It was realizing that uninstalling an application on macOS doesn't necessarily mean you've removed everything associated with that application.

Apps can leave substantial amounts of data behind in places like:

~/Library/Application Support ~/Library/Group Containers ~/Library/Containers ~/Library/Caches

And some of that can end up contributing to the mysterious System Data number you see in macOS Storage.

To be fair, not everything inside System Data is junk. During this same investigation, I found things like my Claude local VM, browser data, developer caches and other files that I actually use and therefore shouldn't delete.

That's why I think the better approach isn't:

"How do I delete System Data?"

It's:

"What exactly is macOS counting as System Data on my machine?"

Once I started asking that question — and got ChatGPT to help me trace the folders one by one — the mystery disappeared pretty quickly.

In my case, 43GB of it was basically two ghosts from apps I thought I'd already deleted.