A Disk Image or DMG file is like a virtual floppy disk that is used to distribute Mac software.
A DMG file (short for Disk Image) is a macOS disk image file format used to distribute software and other files on Apple computers. It is similar to the ISO format used on Windows but is specifically designed for macOS.
TABLE OF CONTENTS
What popular applications download on a DMG file?
Web Browsers:
Google Chrome – One of the most widely used web browsers.
They keep applications self-contained.They can include installation instructions.They allow compression to save space.They offer encryption for secure software distribution.
What are key Features of a DMG File:
Software Distribution: macOS applications are often distributed in DMG files because they can contain the application, installation instructions, and licensing information.
Disk Image Format: DMG files act like a virtual disk, meaning they must be mounted before accessing their contents.
Compression & Encryption: DMG files can be compressed to reduce size and encrypted to protect sensitive data.
Read-Only or Read-Write: DMG files can be created as read-only for distribution or as read-write for storing data.
How do I use a DMG File?
Opening a DMG File: Simply double-click the DMG file on macOS, and it will mount like a virtual drive in Finder.
Installing an Application: If the DMG contains a macOS app, drag and drop the app into the Applications folder.
Ejecting the DMG: After use, unmount it by dragging it to the Trash or right-clicking and selecting “Eject.”
What happens if I try to run the application without moving it off the DMG to the Applications folder?
If you try to run an application directly from the DMG file without moving it to the Applications folder, the following can happen:
1. The Application Will Still Open, But It’s Temporary
Most applications will launch normally from the DMG file.
However, since the DMG is a mounted virtual disk, the app is technically running from a temporary location.
2. The DMG Must Stay Mounted
If you eject the DMG file (by dragging it to the Trash or right-clicking and selecting “Eject”), the application will no longer be accessible.
Every time you restart your Mac, you’ll have to re-mount the DMG and launch the app again.
3. Performance and Updates Issues
Running an application from the DMG can cause slower performance because it isn’t installed properly.
The app might not be able to update itself correctly since it’s not in the Applications folder.
4. Permissions and Security Warnings
macOS may warn you that the app is from an “unidentified developer” or ask for permission to run it.
Some apps may not function properly due to sandboxing restrictions.
5. Missing System Integration
The app may not get full system access, including features like:
Dock integration
Spotlight search indexing
Default app settings
Recommended Action: Move It to Applications
To ensure a smooth experience:
Drag the app from the DMG window to the Applications folder.
Eject the DMG file to free up system resources.
Open the app from the Applications folder as normal.
Can You Open DMG Files on Windows?
By default, Windows does not support DMG files, but third-party tools like 7-Zip, DMG Extractor, or HFSExplorer can extract their contents.
However it is important to note Applications distributed on DMGs are typically Mac based and will generally not function in the Windows environment.
Why would I want to create a DMG?
Creating a DMG (Disk Image) file on macOS has several practical benefits. Here are the main reasons why you might want to create one:
1. Software Distribution
If you are a developer or software distributor, a DMG file is the standard way to package macOS applications.
It allows you to include an application, a background image, and installation instructions in a clean, professional package.
Example: Many macOS apps like Google Chrome, VLC, and Adobe Photoshop are distributed as DMG files.
2. Secure Storage & Encryption
DMG files can be encrypted, making them useful for storing sensitive files (e.g., passwords, financial records, or personal documents).
How? When creating the DMG, choose 128-bit or 256-bit AES encryption in Disk Utility.
Example: You can store confidential work files in an encrypted DMG instead of relying on third-party encryption tools.
3. File Compression & Archiving
DMG files can be compressed, which reduces file size while preserving the original structure.
Useful for archiving large folders, projects, or backups without losing organization.
Example: If you need to send multiple files or folders via email, a compressed DMG reduces the size while keeping everything together.
4. Creating a Bootable Installer
You can create a bootable DMG file to reinstall macOS or troubleshoot issues.
Common use cases:
Installing macOS on multiple computers without re-downloading.
Tech support or IT professionals often create DMG-based recovery tools.
Example: Apple provides macOS installers in DMG format for clean installations.
5. Read-Only DMG for Integrity
DMG files can be set to read-only, preventing modifications to the contained files.
Useful for distributing documents, software, or files without the risk of changes or corruption.
Example: You can distribute legal documents, reports, or software installers in a locked DMG.
6. Creating a Virtual Disk for Temporary Storage
A DMG file acts as a virtual disk, which you can mount and use for temporary file storage.
Unlike a folder, a DMG can be ejected when not in use, keeping your files hidden.
Example: If you work with sensitive client files, you can mount the DMG only when needed.
7. Cross-System File Transfers
While DMG is a macOS format, Windows users can extract DMG files using tools like 7-Zip or DMG Extractor.
This makes it a convenient way to package and share files across different operating systems.
When Should You Use a DMG Instead of a ZIP File?
Feature
DMG
ZIP
Software Distribution
✅ Yes
❌ No
Encryption
✅ Yes
❌ No
Read-Only Protection
✅ Yes
❌ No
Bootable System Creation
✅ Yes
❌ No
Compression
✅ Yes
✅ Yes
Cross-Platform Compatibility
❌ No (macOS only)
✅ Yes (macOS & Windows)
Creating a DMG file is useful for software distribution, encryption, compression, backups, and system recovery. If you’re a developer, IT professional, or privacy-conscious user, a DMG can provide security and organization that standard folders or ZIP files cannot.
How do I Create a DMG File?
Creating a DMG (Disk Image) file on macOS is simple using Disk Utility or the Terminal. Below are the methods you can use:
Method 1: Using Disk Utility (Graphical Interface)
Open Disk Utility
Press Command (⌘) + Space and type “Disk Utility”, then press Enter.
Go to File > New Image > Blank Image
In the top menu, select File > New Image > Blank Image.
Configure DMG Settings
Save As: Choose a name for the DMG file.
Where: Select where to save the DMG (e.g., Desktop).
Name: This is the name that appears when the DMG is mounted.
Size: Choose a size for your DMG file (e.g., 100 MB, 1 GB).
Format: Use Mac OS Extended (Journaled) or APFS (for modern macOS).
Encryption (Optional): You can choose 128-bit or 256-bit AES encryption for security.
Image Format: Choose Read/Write Disk Image if you want to add files later, or Compressed if finalizing.
Click “Save”
This will create a blank DMG file in the location you specified.
Mount and Add Files
Double-click the DMG to mount it, then drag and drop files into it.
Eject and Share
Once you’re done, eject the DMG. It is now ready to distribute.
Method 2: Using Terminal (Command Line)
For more control, you can create a DMG file using Terminal.
Open Terminal
Press Command (⌘) + Space, type “Terminal”, and press Enter.
Run the Following Command:shCopyEdithdiutil create -size 100m -fs HFS+J -volname "MyDisk" MyDisk.dmg
-size 100m → Creates a 100 MB DMG file (change size as needed).
-fs HFS+J → Formats it as Mac OS Extended (Journaled).
-volname "MyDisk" → This is the name when mounted.
MyDisk.dmg → This is the output DMG filename.
Mount the DMG:shCopyEdithdiutil attach MyDisk.dmg
Copy Files into It:shCopyEditcp -R /path/to/files /Volumes/MyDisk/
Unmount and Finalize:shCopyEdithdiutil detach /Volumes/MyDisk
