Talk:Apple Disk Image

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Bold text== Incompatibilities in dmg introduced across Mac OS X 10.1-5 generations == Someone knowledgable needs to add the unpleasant reality of how a dmg created on Intel-Mac fails to mount on 10.1 and 10.2 systems, unless steps are taken to ensure that it does, which gets neglected routinely these days.

For the disk to be mountable on 10.1 and 10.2, it needs to be generated with the hdiutil -fs HFS+ command, switch and argument -- when creating the image on the Intel-Mac.

Else, the 10.2 system, for example, will not be able to mount it, displaying the puzzling catch-all "Error 95" to the user, which is also caused the image was not completely downloaded or got corrupted.

But the real problem is systematic -- the disk's "magic" went unrecognized.

When attepting to mount (attach) such a disk on a 10.2 from command line, say:

hdiutil attach -verbose o950s_4844.dmg (not to pick on Opera, but hey...) you will get output revealing why the disk won't mount, while, in this case, you will be presented in the middle of things with a licence agreement, and if you say Y, it still won't mount. Here's a capture of the whole thing:

marek@[~/Desktop] 69: /usr/bin/hdiutil attach -verbose o950s_4844.dmg 
hdiutil attach: DILoadDriver: checking for disk image driver

DI_kextExists: cannot find IOHDIXController object
hdiutil attach: DILoadDriver: DI_kextExists() returned 0xE00002C0 (-536870208)

hdiutil attach: DILoadDriver: loading disk image driver

hdiutil attach: DILoadDriver: checking again for disk image driver

hdiutil attach: DILoadDriver: DI_kextExists() returned 0x00000000 (0)

DIBackingStoreInstantiatorProbe: interface  0, score      100, CBSDBackingStore
DIBackingStoreInstantiatorProbe: interface  1, score    -1000, CHTTPBackingStore
DIBackingStoreInstantiatorProbe: interface  2, score    -1000, CRAMBackingStore
DIBackingStoreInstantiatorProbe: interface  3, score      100, CCarbonBackingStore
DIBackingStoreInstantiatorProbe: interface  4, score    -1000, CDevBackingStore
DIBackingStoreInstantiatorProbe: interface  5, score    -1000, CCURLBackingStore
DIBackingStoreInstantiatorProbe: selecting CBSDBackingStore
DIFileEncodingInstantiatorProbe: interface  0, score    -1000, CMacBinaryEncoding
DIFileEncodingInstantiatorProbe: interface  1, score    -1000, CAppleSingleEncoding
DIFileEncodingInstantiatorProbe: interface  2, score    -1000, CEncryptedEncoding
DIFileEncodingInstantiatorProbe: nothing to select.
DIFileEncodingInstantiatorProbe: interface  0, score      900, CUDIFEncoding
DIFileEncodingInstantiatorProbe: selecting CUDIFEncoding
DIFileEncodingNewWithBackingStore: CUDIFEncoding
hdiutil attach: DIFileEncodingNewWithBackingStore: instantiator returned 0

DIFileEncodingInstantiatorProbe: interface  0, score    -1000, CSegmentedNDIFEncoding
DIFileEncodingInstantiatorProbe: interface  1, score    -1000, CSegmentedUDIFEncoding
DIFileEncodingInstantiatorProbe: interface  2, score    -1000, CSegmentedUDIFRawEncoding
DIFileEncodingInstantiatorProbe: nothing to select.
DIDiskImageInstantiatorProbe: interface  0, score        0, CDARTDiskImage
DIDiskImageInstantiatorProbe: interface  1, score        0, CDiskCopy42DiskImage
DIDiskImageInstantiatorProbe: interface  2, score    -1000, CNDIFDiskImage
DIDiskImageInstantiatorProbe: interface  3, score     1000, CUDIFDiskImage
DIDiskImageInstantiatorProbe: interface  4, score    -1000, CRawDiskImage
DIDiskImageInstantiatorProbe: interface  5, score     -100, CShadowedDiskImage
DIDiskImageInstantiatorProbe: interface  6, score        0, CSparseDiskImage
DIDiskImageInstantiatorProbe: interface  7, score    -1000, CCFPlugInDiskImage
DIDiskImageInstantiatorProbe: selecting CUDIFDiskImage
DIDiskImageNewWithBackingStore: CUDIFDiskImage
hdiutil attach: DIDiskImageNewWithBackingStore: instantiator returned 0

Opera Browser Information: LICENSE.TXT
=========================================== Copyright (C) Opera
Software 1995-2008

<snip>

Contact us:  <http://www.opera.com/contact/>

Agree Y/N? y
hdiutil attach: mounting "o950s_4844.dmg" failed: no mountable file systems.
marek@[~/Desktop] 70: 

--216.80.74.216 (talk) 21:31, 31 May 2008 (UTC)

Reminds me of the history of ms windows msbackup, qic, ntbackup, etc. at most extreme, a win98 patch destroyed ability to restore archives. 2z2z (talk) 17:06, 17 November 2008 (UTC)

Disk image?

Beyond nomenclature, in what way is this file format a disk image? It sounds more like an archiving format along the lines of zip/rar. Ham Pastrami (talk) 13:54, 24 July 2008 (UTC)

Disk images are in essence archive formats with the goal to replicate a file system structure or sector layout accurately. Standard archive format on the other hand only cares about the files with emphasize on reducing size and maintaining compatibility, etc. --Voidvector (talk) 20:26, 24 September 2008 (UTC)

from googling, seems dmg may optionally be compressed or encrypted, adding complications to work with dmg. Bootable dmg is a whole 'nother headache. 2z2z (talk) 16:59, 17 November 2008 (UTC)

encryption is a whole another matter but I suspect the formats (except the very old ADC) are very standard, it should be trivial to uncompress a bz2/gzip compressed disk image. If you run BSD/Linux "file" command on them, it sees them as bz2 or gz compressed data. Ilgaz (talk) 21:53, 30 April 2009 (UTC)

.dmg format images are disk images in that they contain arbitrary block data, which can be directly mapped as a block device through proper drivers. There is no restriction on what kind of file system / partition system you put on a .dmg image, although HFS+ wrapped in an Apple Partition Map is the most common configuration. You can also put CD images with ISO9660 file systems inside .dmgs. --Unsound (talk) 10:23, 30 November 2008 (UTC)

It is absolutely a disk image, down to the partition map and in fact OS X (hdiutil) can image disks that it can't read at all (because it has no clue about fs) and effectively restore them. Of course, it is not recommended. For example "over the network" time machine backups are actually disk images having their very own partition map and even journal. One can also defragment large disk images or perform maintenance on them with regular tools, just like a real disk. Ilgaz (talk) 21:50, 30 April 2009 (UTC)

dmg2iso binary, windows

btw, dmg2iso binary is either inoperative or usually buggy on win (xp at least). 2z2z (talk) 16:57, 17 November 2008 (UTC)

I can recommend DMGExtractor, a free software Java utility that I wrote by studying dmg2iso and trying to correct bugs in it. It has evolved a lot since then and should handle most .dmg disk images (though not the rare UDCO type, using an Apple proprietary compression algorithm). --Unsound (talk) 10:27, 30 November 2008 (UTC)

dmg from folder

i wonder if this method is "obsolete" in that UI can now make Dmg from folder? 2002 post with similar goal: http://www.macosxhints.com/article.php?story=20020311215452999 2z2z (talk) 17:24, 17 November 2008 (UTC)

for a long time, at least since OS X 10.3.x, Disk Utility (in /Applications/Utilities) can make images from folders, with GUI available in "File"-->"New"--->"Image from Folder" menu of tool. Ilgaz (talk) 22:03, 30 April 2009 (UTC)

Really undocumented (proprietary) format?

I see the article mentions disk image format as something "secret", e.g. hidden specs and I wonder if it is really the case? I have heard similar claims about HFS+ (and journaled variant) and yet we have commercial vendors releasing sector level defragmenter, open source developers releasing "HFS Debug" type utilities and I am not sure they spent any time to "hack" the format rather than reading the official documentation and open source site of Apple.

Are we absolutely sure it is proprietary or perhaps the foreign systems such as Windows live their own (core level) issues with DMG files such as having no free HFS+ reader and not being able to read the actual filesystem itself properly?

It looks like someone was really wanting to use the term proprietary on an Apple article to me.

Even if it evolves (indeed it does, with each major OS update), as long as changes documented by Apple, one can't call it propertiary. Ilgaz (talk) 22:15, 30 April 2009 (UTC)