Why does my SSD suddenly lose 20 GB of free space and then get it back?

Start by checking which filesystem is filling up. Run df -h when space is normal and again during a drop. If the same filesystem changes, use a disk usage analyzer or du to compare large directories on that filesystem. Temporary files, application caches and logs are worth checking.

If the space disappears and returns quickly, you may need to watch while it's happening. A file can also stay allocated after it's deleted if a running process still has it open. On Linux, sudo lsof +L1 can help find those files.

Since it happened while drawing, check whether the drawing app uses temporary files or a disk cache, especially with large images, lots of layers or undo history. Its settings or documentation may show where those files go. That is a lead to investigate, not proof the app caused it.

RAM and disk space are different things, but memory pressure can still affect disk use if swap is enabled. A swap file can exist even without a swap partition. Run swapon --show to see what's active, and check its size before assuming it's responsible.

The drive's age makes a health check sensible, but fluctuating free space alone doesn't establish that the SSD is failing. Back up anything you can't afford to lose, then check its SMART health data with Disks or smartctl. Also look for disk errors in the system logs, especially around the slowdown.