Why 20+ GB?
When usable free disk space drops below a system threshold, macOS will stop a screen recording in progress — without warning. There's nothing any recording app can do once that happens. Your recording ends mid-take.
20 GB sounds like a lot, especially compared to most files you save day-to-day. But screen recording is unusual: it produces a continuous stream of video and audio that grows by hundreds of MB per minute on every recorded source.
On a MacBook Air it may need more than 20 GB as due to the machine being thermal (fan-less) the system is more aggressive with throttling.
20 GB is the number we use because it gives you enough room to:
Record a normal session (recordings are large)
Stay above macOS's own termination threshold
Account for Time Machine local snapshots (see below)
"But Storage says I have 100 GB free"
This is the most common point of confusion. The "Available" figure in System Settings → General → Storage is not always the same as actually-free space.
A large part of what looks free is often held by services like Time Machine local snapshots — hourly backups Time Machine keeps on your startup disk. macOS will free that space if needed, but not always quickly enough. So the headline "Available" number can be much bigger than what's actually usable right now.
Why we warn rather than block
You can press "Continue Anyway". The pre-flight warning is a heads-up that conditions are risky, not a hard stop. But if your recording cuts out partway through, low disk space is the first thing to check.
Still confused?
Different Macs have different size SSDs — a 256 GB MacBook Air feels much fuller than a 1 TB MacBook Pro at the same percentage. If you regularly hit this warning on a smaller Mac, the long-term fix is usually moving large media (Photos library, video projects, big Downloads) off the startup drive and onto external storage or iCloud.