Intermittent disk space loss on iMac with macOS Sequoia 15.8.2

Hi,

Intermittent disk space loss on macOS Sequoia 15.8.2 is typically caused by background system subsystems that create temporary files, caches, or snapshots, and fail to purge them immediately. In macOS Sequoia, Apple reclassified many hidden files into the System Data category, which can balloon and shrink randomly.

The primary culprits behind this issue on your iMac include:

1. APFS Local Time Machine Snapshots

Even if your external Time Machine drive is disconnected, macOS takes hourly background filesystem snapshots. If you recently deleted large files or apps, APFS preserves those data blocks in a hidden local snapshot until it expires (usually after 24 hours) or until disk pressure forces a purge.

• The Fix: Connect your Time Machine backup drive and let a backup cycle complete to clear old snapshots. Alternatively, open Terminal and force-thin them by typing:
• sudo tmutil thinlocalsnapshots / 10000000000 4
• (This requests the system to immediately purge up to 10 GB of local snapshots with urgent priority).

2. The mediaanalysisd and idleassetsd Caching Loops

A known issue in macOS Sequoia involves background processes like com.apple.mediaanalysisd (which scans photos/videos for faces and objects) and idleassetsd (which downloads dynamic wallpapers and screensavers). These processes can loop, rapidly writing gigabytes of data to your drive, then clearing them later.

• The Fix: Go to System Settings > Wallpapers and switch your background to a static, pre-installed Apple wallpaper rather than a dynamic aerial video. To wipe out a bloated media analysis cache, clear the contents of ~/Library/Caches/com.apple.mediaanalysisd/.

3. Spotlight Knowledge Events & Mail Indexing

Upgrades to Sequoia can cause the CoreSpotlight indexing daemon to aggressively journal Mail and Messages data. The folder ~/Library/Metadata/SpotlightKnowledgeEvents can unexpectedly bloat to dozens of gigabytes.

• The Fix: If you notice your iMac getting warm or your space disappearing while Mail is open, go to System Settings > Siri & Spotlight, scroll down to the Search Results list, and temporarily uncheck Mail and Messages.

4. Overburdened Purgeable Storage (The "Disk Pressure" Trick)

macOS retains gigabytes of data as "Purgeable Space," promising to delete it when you run low. However, the OS often waits until the very last second, causing "Disk Full" errors before triggering its cleanup routine.

• The Fix: You can trick macOS into releasing this space instantly. Open Terminal and run:
• dd if=/dev/zero of=~/purge_dummy_file bs=100m
• Let it run for a minute until a "Disk Almost Full" warning pops up. This triggers macOS to purge all non-essential caches. Once the alert shows up, press Control + C in Terminal to stop it, then run:
• rm ~/purge_dummy_file

5. Clear Caches via Safe Mode

If the loss is erratic, a corrupted system cache file might be the anchor.

• The Fix: Shut down your iMac. If it is an Intel-based iMac, turn it on and immediately hold the Shift key. If it is an Apple Silicon iMac (M1/M2/M3), press and hold the power button until you see the startup options screen, select your disk, hold Shift, and click "Continue in Safe Mode."
• Let the Mac sit for 5 minutes in Safe Mode (which runs automatic directory checks and purges system caches), then restart normally.

You're out of free disk space. The "available" value is just nonsense. But in your case, since you're living on that bleeding edge, you do see some interesting activity. Once your "available" gets to zero, the system purges some data and gets you back to 14 GB. Curiously, that's pretty much the opposite of what the general idea of how "available" storage is supposed to work.

But that's all irrelevant. You need to delete about 100 GB of data. Maybe you could get by with deleting 50 GB.

And ironically enough, deleting files will actually reduce the amount of free storage. But after a day or two, the automated storage cleanup task will eventually run and recover that storage. You're doing pretty good to force that thing to run like that. But that's dangerous. You're really, really close to catastrophic failure and data corruption.

thank you I will try these issues and says it something for that