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OS X dmg files – Disk Utility usually creates them read-only

Posted by jpluimers on 2017/11/27

While recovering the backups of a Retina MacBook Pro, I needed to know if I could mount a Disk Utility backup image on read-only mode.

Finding that info was a tad difficult as most results were about making dmg files read-write. This already hinted me in that most dmg files were readonly.

The main tool for handling dmg files is hdiutil. for which you can get more info here:

The hdiutil command imageinfo will output a Format: line having the kind of dmg image:

  • UDRW – UDIF read/write image
  • UDRO – UDIF read-only image
  • UDCO – UDIF ADC-compressed image
  • UDZO – UDIF zlib-compressed image
  • UDBZ – UDIF bzip2-compressed image (OS X 10.4+ only)
  • UFBI – UDIF entire image with MD5 checksum
  • UDRo – UDIF read-only (obsolete format)
  • UDCo – UDIF compressed (obsolete format)
  • UDTO – DVD/CD-R master for export
  • UDxx – UDIF stub image
  • UDSP – SPARSE (grows with content)
  • UDSB – SPARSEBUNDLE (grows with content; bundle-backed)
  • RdWr – NDIF read/write image (deprecated)
  • Rdxx – NDIF read-only image (Disk Copy 6.3.3 format)
  • ROCo – NDIF compressed image (deprecated)
  • Rken – NDIF compressed (obsolete format)
  • DC42 – Disk Copy 4.2 image

All compressed formats seem to be read-only and of course all read-only formats. Which then basically leaves only these as (potentialy) read-write image types:

  • UDRW
  • UFBI
  • UDSP
  • UDSB
  • RdWr
  • DC42

References and further reading:

–jeroen

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