The icon depicts an internal hard drive within a generic file icon. | |
Filename extension | |
---|---|
Internet media type | application/x-apple-diskimage |
Uniform Type Identifier (UTI) | com.apple.disk-image |
Developed by | Apple Inc. |
Type of format | Disk image |
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- The Amazing Spider-Man is an Action game developed by Gameloft. BlueStacks app player is the best platform (emulator) to play this Android game on your PC or Mac for an immersive gaming experience. New York City is in danger once again.
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Apple[1]Disk Image is a disk image format commonly used by the macOS operating system. When opened, an Apple Disk Image is mounted as a volume within the Macintosh Finder.
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An Apple Disk Image can be structured according to one of several proprietary disk image formats, including the Universal Disk Image Format (UDIF) from Mac OS X and the New Disk Image Format (NDIF) from Mac OS 9. An Apple disk image file's name usually has '.dmg' as its extension.
Features[edit]
Apple Disk Image files are published with a MIME type of application/x-apple-diskimage.
Different file systems can be contained inside these disk images, and there is also support for creating hybrid optical media images that contain multiple file systems.[1] Some of the file systems supported include Hierarchical File System (HFS), HFS Plus, File Allocation Table (FAT), ISO9660 and Universal Disk Format (UDF).[1][2]
Apple Disk Images can be created using utilities bundled with Mac OS X, specifically Disk Copy in Mac OS X v10.2 and earlier and Disk Utility in Mac OS X v10.3 and later. These utilities can also use Apple disk image files as images for burning CDs and DVDs. Disk image files may also be managed via the command line interface using the hdiutil utility.[3]
In Mac OS X v10.2.3, Apple introduced Compressed Disk Images[4] and Internet-Enabled Disk Images for use with the Apple utility Disk Copy, which was later integrated into Disk Utility in 10.3. The Disk Copy application had the ability to display a multilingual software license agreement before mounting a disk image. The image will not be mounted unless the user indicates agreement with the license.[5]
An Apple Disk Image allows secure password protection as well as file compression, and hence serves both security and file distribution functions; such a disk image is most commonly used to distribute software over the Internet.
History[edit]
Apple originally created its disk image formats because the resource fork used by Mac applications could not easily be transferred over mixed networks such as those that make up the Internet. Even as the use of resource forks declined with Mac OS X, disk images remained the standard software distribution format. Disk images allow the distributor to control the Finder's presentation of the window, which is commonly used to instruct the user to copy the application to the correct folder.
A previous version of the format, intended only for floppy disk images, is usually referred to as 'Disk Copy 4.2' format, after the version of the Disk Copy utility that was used to handle these images.[1] A similar format that supported compression of floppy disk images is called DART.[1][6]
New Disk Image Format (NDIF) was the previous default disk image format in Mac OS 9,[1] and disk images with this format generally have a .img (not to be confused with raw .img disk image files) or .smi file extension. Files with the .smi extension are actually applications that mount an embedded disk image, thus a 'Self Mounting Image', intended only for Mac OS 9 and earlier.[7][2]
Universal Disk Image Format (UDIF) is the native disk image format for Mac OS X. Disk images in this format typically have a .dmg extension.[1]
File format[edit]
Apple has not released any documentation on the format, but attempts to reverse engineer parts of the format have been successful. The encrypted layer was reverse engineered in an implementation called VileFault (a spoonerism of FileVault).[8]
Apple disk image files are essentially raw disk images (i.e. contain block data) with some added metadata, optionally with one or two layers applied that provide compression and encryption. In hdiutil, these layers are called CUDIFEncoding and CEncryptedEncoding.[1]
UDIF supports ADC (an old proprietary compression format by Apple), zlib, bzip2 (as of Mac OS X v10.4), and LZFSE (as of Mac OS X v10.11)[9] compression internally.
Metadata[edit]
The UDIF metadata is found at the end of the disk image following the data. This trailer can be described using the following C structure.[10] All values are big-endian (PowerPC byte ordering)
The XML plist contains a blkx
(blocks) key, with information about how the preceding data fork is allocated. The main data is stored in a base64 block, using tables identified by the magic 'mish'
. This 'mish'
structure contains a table about blocks of data and the position and lengths of each 'chunk' (usually only one chunk, but compression will create more).[10] The data and resource fork information is probably inherited from NDIF.
Encryption[edit]
The encryption layer comes in two versions. Version 1 has a trailer at the end of the file, while version 2 (default since OS X 10.5) puts it at the beginning. Whether the encryption is a layer outside of or inside of the blkx
metadata (UDIF) is unclear from reverse engineered documentation, but judging from the vfcrack
demonstration it's probably outside.[8]
Utilities[edit]
There are few options available to extract files or mount the proprietary Apple Disk Image format. Some cross-platform conversion utilities are:
- dmg2img was originally written in Perl; however, the Perl version is no longer maintained, and the project was rewritten in C. It extracts the raw disk image from a DMG, without handling the file system inside. UDIF ADC-compressed images (UDCO) have been supported since version 1.5.[11]
- DMGEXtractor is written in Java with GUI, and it supports more advanced features of dmg including AES-128 encrypted images but not UDCO images.[12]
- The Sleuth Kit. Only handles uncompressed DMG format, HFS+, and APFS.
Most dmg files are unencrypted. Because the dmg metadata is found in the end, a program not understanding dmg files can nevertheless read it as if it was a normal disk image, as long as there is support for the file system inside. Tools with this sort of capacity include:
- Cross-platform: 7-zip (HFS/HFS+), PeaZip (HFS/HFS+).
- Windows: UltraISO, IsoBuster, MacDrive (HFS/HFS+).[13]
- Unix-like: cdrecord and
mount
(e.g.mount -o loop,ro -t hfsplus imagefile.dmg /mnt/mountpoint
).[14][15]
Tools with specific dmg support include:
- Windows:
- Transmac can handle both UDIF dmgs and sparsebundles, as well as HFS/HFS+ and APFS. It is unknown whether it handles encryption.[16] It can be used to create bootable macOS installers under Windows.[17]
- A free Apple DMG Disk Image Viewer also exists, but it is unknown how much what it actually supports.[18]
- Unix-like:
- darling-dmg is a FUSE module enabling easy DMG file mounting on Linux. It supports UDIF and HFS/HFS+.[19]
See also[edit]
References[edit]
- ^ abcdefgh'hdiutil(1) Mac OS X Manual Page'. Archived from the original on 2016-05-14. Retrieved 2016-05-14.
- ^ ab'Mac OS X: Using Disk Copy disk image files'. Archived from the original on 2013-12-19. Retrieved 2013-04-25.
- ^
hdiutil(1)
– Darwin and macOS General Commands Manual - ^'Re: Some apps refuse to launch in 10.2.8! (OT, but very important)'. Archived from the original on 2014-01-17.
- ^'Guides'. Apple. Archived from the original on 2009-03-06. Retrieved 2010-10-27.
- ^'DART 1.5.3: Version Change History'. Archived from the original on 2013-12-19. Retrieved 2013-04-25.
- ^'Software Downloads: Formats and Common Error Messages'. Archived from the original on 2010-12-24. Retrieved 2009-05-06.
- ^ ab'VileFault'. 2006-12-29. Archived from the original on 2007-01-09. Retrieved 2010-10-27.
- ^Michael Tsai (2015-10-07). 'LZFSE Disk Images in El Capitan'. Archived from the original on 2017-04-09. Retrieved 2017-04-09.
- ^ ab'Demystifying the DMG File Format'. Archived from the original on 2013-03-17.
- ^'dmg2img'. Archived from the original on 29 November 2010. Retrieved 2010-10-27.
- ^'DMGExtractor'. Archived from the original on 2011-01-02. Retrieved 2011-01-03.
- ^MacDrive Features / Boot Camp / System Requirements /. 'MacDrive Home page'. Mediafour. Archived from the original on 4 October 2010. Retrieved 2010-10-27.
- ^'How To Convert DMG To ISO in Windows, Linux & Mac'. Archived from the original on 2010-03-07.
- ^'Convert DMG To ISO using PowerISO'. Archived from the original on 2009-05-02. Retrieved 2009-02-15.
- ^'About TransMac for Windows'. www.acutesystems.com.
- ^'Convert'. www.winytips.com. winytips. Retrieved 5 May 2020.
- ^Olivia Dehaviland (2015-03-03). 'Apple DMG Disk Image Viewer'. DataForensics.org. Archived from the original on 2015-04-02. Retrieved 2015-03-10.
- ^'darling-dmg'. darling-dmg. Retrieved 29 March 2015.
External links[edit]
- Apple Developer Connection A Quick Look at PackageMaker and Installer
- O'Reilly Mac DevCenter Tip 16-5. Create a Disk Image from a Directory in the Terminal
Optimized Storage helps you save storage space space by storing your content in iCloud and making it available on demand:
- When storage space is needed, files, photos, movies, email attachments, and other files that you seldom use are stored in iCloud automatically.
- Each file stays right where you last saved it, and downloads when you open it.
- Files that you've used recently remain on your Mac, along with optimized versions of your photos.
If you haven't yet upgraded to macOS Sierra or later, learn about other ways to free up storage space.
Find out how much storage is available on your Mac
Choose Apple menu > About This Mac, then click Storage. Each segment of the bar is an estimate of the storage space used by a category of files. Move your pointer over each segment for more detail.
Click the Manage button to open the Storage Management window, pictured below.
Manage storage on your Mac
The Storage Management window offers recommendations for optimizing your storage. If some recommendations are already turned on, you will see fewer recommendations.
Store in iCloud
Click the Store in iCloud button, then choose from these options:
- Desktop and Documents. Store all files from these two locations in iCloud Drive. When storage space is needed, only the files you recently opened are kept on your Mac, so that you can easily work offline. Files stored only in iCloud show a download icon , which you can double-click to download the original file. Learn more about this feature.
- Photos. Store all original, full-resolution photos and videos in iCloud Photos. When storage space is needed, only space-saving (optimized) versions of photos are kept on your Mac. To download the original photo or video, just open it.
- Messages. Store all messages and attachments in iCloud. When storage space is needed, only the messages and attachments you recently opened are kept on your Mac. Learn more about Messages in iCloud.
Storing files in iCloud uses the storage space in your iCloud storage plan. If you reach or exceed your iCloud storage limit, you can either buy more iCloud storage or make more iCloud storage available. iCloud storage starts at 50GB for $0.99 (USD) a month, and you can purchase additional storage directly from your Apple device. Learn more about prices in your region.
Optimize Storage
Click the Optimize button to save space by automatically removing watched movies and TV shows. When storage space is needed, movies or TV shows that you purchased from Apple and already watched are removed from your Mac. Click the download icon next to a movie or TV show to download it again.
Your Mac will also save space by keeping only recent email attachments on this Mac when storage space is needed. You can manually download any attachments at any time by opening the email or attachment, or saving the attachment to your Mac.
Optimizing storage for movies, TV shows, and email attachments doesn't require iCloud storage space.
Empty Trash Automatically
Empty Trash Automatically permanently deletes files that have been in the Trash for more than 30 days.
Reduce Clutter
Reduce Clutter helps you identify large files and files you might no longer need. Click the Review Files button, then choose any of the file categories in the sidebar, such as Applications, Documents, Music Creation, or Trash.
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You can delete the files in some categories directly from this window. Other categories show the total storage space used by the files in each app. You can then open the app and decide whether to delete files from within it.
Learn how to redownload apps, music, movies, TV shows, and books.
Where to find the settings for each feature
The button for each recommendation in the Storage Management window affects one or more settings in other apps. You can also control those settings directly within each app.
- If you're using macOS Catalina or later, choose Apple menu > System Preferences, click Apple ID, then select iCloud in the sidebar: Store in iCloud turns on the Optimize Mac Storage setting on the right. To turn off iCloud Drive entirely, deselect iCloud Drive.
- If you're using macOS Mojave or earlier, choose Apple menu > System Preferences, click iCloud, then click Options next to iCloud Drive. Store in iCloud turns on the Desktop & Documents Folders and Optimize Mac Storage settings.
- In Photos, choose Photos > Preferences, then click iCloud. Store in iCloud selects iCloud Photos and Optimize Mac Storage.
- In Messages, choose Messages > Preferences, then click iMessage. Store in iCloud selects Enable Messages in iCloud.
- If you're using macOS Catalina or later, open the Apple TV app, choose TV > Preferences from the menu bar, then click Files. Optimize Storage selects 'Automatically delete watched movies and TV shows.'
- In you're using macOS Mojave or earlier, open iTunes, choose iTunes > Preferences from the menu bar, then click Advanced. Optimize Storage selects 'Automatically delete watched movies and TV shows.'
- In Mail, choose Mail > Preferences from the menu bar, then click Accounts. In the Account Information section on the right, Optimize Storage sets the Download Attachments menu to either Recent or None.
Empty Trash Automatically: From the Finder, choose Finder > Preferences, then click Advanced. Empty Trash Automatically selects 'Remove items from the Trash after 30 days.'
Other ways that macOS helps automatically save space
With macOS Sierra or later, your Mac automatically takes these additional steps to save storage space:
- Detects duplicate downloads in Safari, keeping only the most recent version of the download
- Reminds you to delete used app installers
- Removes old fonts, languages, and dictionaries that aren't being used
- Clears caches, logs, and other unnecessary data when storage space is needed
How to free up storage space manually
Even without using the Optimized Storage features described in this article, you can take other steps to make more storage space available:
- Music, movies, and other media can use a lot of storage space. Learn how to delete music, movies, and TV shows from your device.
- Delete other files that you no longer need by moving them to the Trash, then emptying the Trash. The Downloads folder is good place to look for files that you might no longer need.
- Move files to an external storage device.
- Compress files.
- Delete unneeded email: In the Mail app, choose Mailbox > Erase Junk Mail. If you no longer need the email in your Trash mailbox, choose Mailbox > Erase Deleted Items.
Learn more
- The Storage pane of About This Mac is the best way to determine the amount of storage space available on your Mac. Disk Utility and other apps might show storage categories such as Not Mounted, VM, Recovery, Other Volumes, Free, or Purgeable. Don't rely on these categories to understand how to free up storage space or how much storage space is available for your data.
- When you duplicate a file on an APFS-formatted volume, that file doesn't use additional storage space on the volume. Deleting a duplicate file frees up only the space required by any data you might have added to the duplicate. If you no longer need any copies of the file, you can recover all of the storage space by deleting both the duplicate and the original file.
- If you're using a pro app and Optimize Mac Storage, learn how to make sure that your projects are always on your Mac and able to access their files.