Backups & restore points · Speed, progress and the first backup
Why did my backup pause while my Mac was busy?
Short answer: Storage Studio holds backups while your Mac is under heavy load so your work stays smooth. The backup resumes on its own from where it stopped; Back up now runs it anyway.
To keep your Mac responsive. When the machine is under heavy load, Storage Studio holds background work, including backups, and the card says "Paused while your Mac is busy — … It picks up where it left off on its own."
Nothing is lost and there's nothing to do: the backup continues once things calm down. If you need it to run regardless, press Back up now.