Mac storage basics · Memory, swap and speed
Why is my Mac's disk busy even when I'm not doing anything?
Short answer: macOS runs idle-time maintenance: Spotlight indexing, Time Machine, Photos analysis, iCloud syncing and updates. Check Activity Monitor's Disk tab for the process responsible.
macOS does maintenance when you're idle: Spotlight indexing, Time Machine backups, Photos analysing pictures, iCloud syncing, software update checks and app updates. After an update or a big import this can last hours.
Activity Monitor's Disk tab shows which process is responsible. Persistent heavy activity from one ordinary app — not a system service — is worth investigating.