iCloud and cloud storage

Optimise Mac Storage, explained

Updated 2026-07-31 · 4 min read

One checkbox quietly decides whether your files are on your Mac. Worth understanding before trusting it.

What it does

With Optimise Mac Storage on (System Settings → Apple ID → iCloud → Drive settings, and separately in Photos → Settings → iCloud), macOS may replace a file's local contents with a placeholder when space runs low, keeping the full copy only in iCloud.

Eviction is automatic, needs-based and unannounced. You do not choose which files; the system does, generally the least recently used.

What an evicted file looks like

Cloud icon in Finder. Name, size, dates all present — the directory entry is real. Opening it triggers a download. On disk it occupies almost nothing, which is the point.

The consequences worth knowing

• Offline access is not guaranteed. The file you need on the plane may be a placeholder
• Content search breaks for evicted files — nothing local to read. Name search still works
• Backups shrink. Time Machine backs up what is on disk; evicted contents are not.

Your only copy of an evicted file is Apple's

• "Size on disk" arithmetic stops adding up, since reported and occupied sizes diverge

Turning it off

Uncheck it and macOS re-downloads everything — ensure the disk actually fits your iCloud contents first. The Photos version behaves the same for originals.

A reasonable policy

On a small-SSD laptop, leave it on and mark critical folders Keep Downloaded (right-click) so they are never evicted. On a Mac with room, turn it off — local copies mean complete backups and working search, and disk space is what you bought it for.

Common questions

Does Time Machine back up evicted iCloud files?

No — only what is locally present. Evicted files exist solely in iCloud until re-downloaded.

How do I stop specific folders being evicted?

Right-click → Keep Downloaded pins them locally even with Optimise Storage on.