One likely cause of vanishing free space are large snapshots. So how can you tell why a snapshot is large, and how can you avoid it?
Disk Utility
Snapshots are of great value, but can grow large if you don’t keep an eye on them. If you find Time Machine or another backup utility is storing large snapshots, here’s what to do.
We used to repair permissions of the system until SIP. After a short break, we often repaired them again, this time on preference files. Why did Apple cancel that? Should we still try it for fixing problems?
According to macOS Help, safe mode stops some software from loading, and performs a check of the startup disk. Here’s a more detailed and accurate account of what it does.
When you try ejecting a single volume of a bootable volume group on an external disk, other volumes are also ejected even though you chose to keep them mounted.
How to repair a snapshot with an error when it’s one of your backups on a hard disk? Currently this seems impossible, putting all those backups at risk.
Don’t waste time trying to discover which files are still open when you want to check or repair an APFS disk and Disk Utility throws an error. Here’s the solution.
When your backups are stored on hard disks, even though they may use APFS, it’s important to check they’re sound. How to do better than Disk Utility.
A major new version of macOS bringing few big changes, but lots of opportunity for improvements and fixes.
Explains the identifiers, sizes and tidemark data shown for snapshots in Disk Utility, in Monterey.
The Eclectic Light Company 