macOS disk full? Check swap and pending update first

When df shows almost no free space but cache deletions barely move the needle, the culprit is often outside the Data volume. Run diskutil apfs list to see all volumes in the container. The VM volume (disk3s6 /System/Volumes/VM) holds swap and can balloon to tens of GB under memory pressure; the Update mount (disk3s1 /System/Volumes/Update/mnt1) holds the staged macOS update and can be ~18 GB. Neither is safely deleted while running. The fixes are a reboot (purges swap) and installing or canceling the pending Software Update.