“Your startup disk is almost full” — what’s actually taking the space

The warning shows up when you’re already down to a few gigabytes, which means you don’t have much time to figure out what’s actually eating your storage before things start failing to save. The good news: on almost every Mac, the space isn’t going where people assume.

First, see what macOS thinks is using the space

Apple menu → About This Mac → Storage → Manage. This gives you an actual breakdown by category instead of guessing, and it’s the starting point for everything below. The categories that surprise people, in order:

1. “Other” or “System Data” — almost always the biggest surprise

This catch-all category often turns out to be the single largest chunk of used space, and it’s mostly caches: browser caches, app caches, system logs, and temporary files that build up over months or years without anyone clearing them. It’s mostly safe to be large, but if it’s grown to tens of gigabytes, a restart alone clears some of it, and the Storage Management panel’s “reduce clutter” tools handle more.

2. Photos — especially if iCloud Photos is on but not actually optimizing

If you have iCloud Photos enabled with “Optimize Mac Storage” turned off, every photo and video you’ve ever taken on any device may be stored at full resolution locally — which for a library going back years is very often tens of gigabytes on its own. Check: Photos app → Settings → iCloud — and make sure “Optimize Mac Storage” is selected rather than “Download Originals to this Mac,” unless you specifically need full-resolution originals available offline.

3. Old iOS/iPadOS backups and unused apps

If you’ve ever backed up an iPhone or iPad to this Mac via Finder rather than iCloud, those backups sit on the drive indefinitely and are easy to forget about. The Storage Management panel lists them directly and lets you delete old ones in a couple of clicks. Same goes for apps you installed once and never opened again — the Applications category in that same panel sorts by size, which usually surfaces one or two multi-gigabyte apps nobody’s used in a year.

4. The Downloads folder, quietly, for years

Downloads is one of the few folders macOS doesn’t clean up automatically, and it accumulates everything: installers you ran once, PDFs, large attachments, video files someone sent you. It’s worth a five-minute sort-by-size pass in Finder specifically on this folder — people are routinely shocked to find 10–20 GB of forgotten downloads sitting there.

5. Mail attachments and offline email

If your Mail app is set to download every attachment for offline access, and you’ve had the same email account for years, this can add up to a genuine amount of space — particularly for anyone who emails large files regularly. Mail’s own settings let you limit how much gets stored locally versus fetched on demand.

What actually frees up space, in order of effort

1. Turn on “Optimize Mac Storage” for Photos if it isn’t already (biggest single win for most people, zero manual work after).
2. Delete old iOS device backups you don’t need through Storage Management.
3. Sort Downloads by size and clear out what you don’t need.
4. Uninstall apps you haven’t opened in the last year.
5. Empty the Trash (it doesn’t count as freed space until you do — an easy one to forget).

When it’s more than cleanup

If you’ve worked through all of this and you’re still consistently near full, the honest fix is more storage — either an external drive for anything that doesn’t need to live on the internal disk (old projects, media libraries), or, if the machine is older and the internal drive is genuinely small by today’s standards, that’s a conversation about whether the Mac itself still fits how much you’re storing. We can go through this with you directly rather than you guessing at what’s safe to delete — it’s part of our Mac and Windows help across the Upper West Side and Manhattan.