Glossary
Time Machine
macOS's backup system. Local hourly snapshots via APFS plus optional external backups to USB or network destinations.
Time Machine is macOS's backup system. On a modern Mac it runs in two modes:
- Local snapshots — APFS snapshots of the boot volume, taken hourly when on AC power and idle. Kept for 24 hours. Always on, even without an external destination.
- Destination backups — full + differential backups to an external destination (USB drive, Thunderbolt drive, NAS over SMB, or another Mac).
Both use the same underlying APFS snapshot machinery. Creation is O(1) regardless of volume size; the differential is computed at the b-tree XID level so changed-block detection is fast.
Retention bands:
| Age | Kept |
|---|---|
| Last 24 hours | hourly |
| Last month | daily |
| Older | weekly |
Tools:
tmutil— the command-line interface.tmutil snapshot— take a local snapshot now.tmutil listlocalsnapshots /— list current local snapshots.tmutil restore <src> <dst>— restore a single file from a snapshot.
Network destinations work via a sparsebundle disk image mounted as a child APFS volume on the destination's SMB share. Performance is bottlenecked by the sparsebundle format on top of network round-trips.
See also: APFS, and the Time Machine article.