What is the SPARSEBUNDLE file format?
A .sparsebundle is an Apple macOS sparse bundle disk image - a virtual disk container introduced in Mac OS X 10.5 Leopard (2007) alongside Time Machine. Despite appearing as a single icon in Finder, a .sparsebundle is actually a macOS directory bundle (package) containing an Info.plist metadata file, a token file, and a bands/ subdirectory of fixed-size data segments.
The key feature is its band-based storage: the virtual disk is split into 8 MiB band files (since macOS 10.8). Only bands that contain actual data exist on disk, so the image grows only as data is written. When Time Machine backs up to a network share (NAS or AirPort Time Capsule), only the changed band files need to be transferred - making incremental backups far more efficient than with a monolithic .dmg.
The internal filesystem is typically HFS+ or APFS. Optional encryption is available - AES-128 or AES-256 (256-bit since macOS 10.7 Lion) - configurable via hdiutil or Disk Utility.
A .sparsebundle can be mounted by double-clicking in Finder or with the command hdiutil attach. Corruption of any band file or the Info.plist can make the image unmountable; the internal filesystem can be repaired with fsck_hfs or fsck_apfs, though the band structure itself has no self-healing mechanism.
Security & safety
RISK: LOWSparsebundle itself carries no malware risk - it is a container. Contents of a mounted sparsebundle should be treated with the same caution as any disk/filesystem. Encrypted sparsebundles (AES-256) protect data at rest effectively. Caution: do not forcibly unmount or disconnect a NAS while Time Machine is writing - this can corrupt the Info.plist and make the sparsebundle unmountable.
Format details
in a nutshellPrograms that open SPARSEBUNDLE files
Windows1 app
7-Zip
Open-source Can browse the bands/ folder contents but cannot interpret the virtual filesystem - not a true mount. Inspect only. 
macOS3 apps
Disk Utility
Built-in File → Open Disk Image → select .sparsebundle, or double-click in Finder to mount.
DiskImageMounter (macOS built-in)
Built-in Double-click the .sparsebundle in Finder - DiskImageMounter mounts it automatically as a volume.
hdiutil (command-line)
Built-in hdiutil attach /path/to/image.sparsebundle - mounts it as a virtual disk. Use hdiutil verify to check integrity. Technical details
deep spec| Format type | Directory-based sparse virtual disk image (macOS bundle/package) |
| Bundle structure | macOS package directory containing Info.plist, token file, and bands/ subdirectory |
| Band file size | 8 MiB per band (macOS 10.8+); 8.4 MB on older macOS versions |
| Encoding | Binary (band files) + XML text (Info.plist metadata) |
| Byte order | Dependent on internal filesystem; HFS+ and APFS use Apple-native ordering |
| Compression | None by default; optional zlib compression per band (user-selectable at creation) |
| Encryption | Optional AES-128 or AES-256 (256-bit available since macOS 10.7 Lion) |
| Typical size | 1 GB - several TB; physical disk usage grows only as bands are written (sparse) |
| Internal filesystem | HFS+ or APFS (user-selected at creation) |
| Identification | No single magic bytes - identified by directory name ending in .sparsebundle and presence of Info.plist |
| Introduced | Mac OS X 10.5 Leopard (October 26, 2007), with Time Machine launch |
| Primary use case | Time Machine backups to network shares (NAS, AirPort Time Capsule) - only changed bands are transferred per backup cycle |
| Mount command | hdiutil attach <image>.sparsebundle or double-click in Finder |
| Integrity repair | fsck_hfs or fsck_apfs repairs internal filesystem; band structure has no self-healing mechanism |
| Released | 2007 (Mac OS X 10.5 Leopard, with Time Machine) |
SPARSEBUNDLE conversions
Community Q&A
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