Mac storage basics · Disks, APFS and formats

What are disk images (.dmg) and sparse bundles?

Q: What are disk images (.dmg) and sparse bundles?

A disk image is a file that behaves like a drive when you open it. A .dmg has a fixed contents size and is how most apps are distributed. A sparse image or sparse bundle grows as you add files (a sparse bundle stores itself as many small pieces, which suits backups and syncing). You can make one in Disk Utility → File → New Image. Sparse images don't automatically shrink when you delete files inside; Disk Utility's Resize or the hdiutil compact command reclaims the space.

Short answer: A disk image is a file that acts like a drive. A .dmg has fixed contents; sparse images and bundles grow as you add files and need compacting to shrink.

A disk image is a file that behaves like a drive when you open it. A .dmg has a fixed contents size and is how most apps are distributed. A sparse image or sparse bundle grows as you add files (a sparse bundle stores itself as many small pieces, which suits backups and syncing).

You can make one in Disk Utility → File → New Image. Sparse images don't automatically shrink when you delete files inside; Disk Utility's Resize or the hdiutil compact command reclaims the space.

Last reviewed October 1, 2026

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What are disk images (.dmg) and sparse bundles?

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