A DMG file is a disk image — a single file that contains a copy of everything that was on a disk or folder, compressed into one package

On a Mac, DMG stands for "disk image". Think of it as a photograph of a hard drive or USB stick frozen in time. When you open a DMG file, your Mac mounts it as if it were a real disk plugged in, even though it is just one file sitting on your actual hard drive. You will see it appear on your desktop like any other drive, and you can copy files from it, install software from it, or browse through it exactly as you would a physical disk.

DMG files are common on macOS because Apple uses them for software distribution, system backups, and file transfers. If you downloaded an process from the internet on a Mac, it probably came inside a DMG file. Your Mac may also create DMG files automatically when you use certain backup tools or when you prepare files to send to someone else.

Key Takeaways

  • A DMG file is a compressed container that holds the exact contents of a disk or folder, and your Mac treats it like a mounted drive when you open it.
  • DMG files are created by macOS backup tools, used by software publishers to distribute applications, and sometimes created manually to compress and organize files.
  • Opening a DMG file does not extract it or use up extra storage — your Mac straightforward mounts it temporarily so you can access what is inside.
  • You can copy files out of a DMG, but changes you make inside a mounted DMG are usually lost when you eject it unless the DMG is read-write format.
  • DMG files are Mac-specific and will not open on Windows or Linux without special software.

Why your Mac created a DMG file

Your Mac most likely created a DMG file through one of three routes. The first is automatic backup software — if you use Time Machine, Carbon Copy Cloner, or similar tools, they often save backups as DMG files. The second is manual compression: you may have right-clicked a folder, chosen "Compress", and your Mac created a DMG without you realizing it. The third is preparation for transfer — if you used file-sharing software or prepared files to send to another Mac user, the system may have wrapped them in a DMG.

Check your Downloads folder and Desktop first. DMG files created by accident often end up in these places. If you find one there and do not remember creating it, it probably came from downloading an process installer or a file someone sent you.

How to open a DMG file

Double-click the DMG file. That is the entire process. Your Mac will mount it, and a new disk icon will appear on your desktop and in Finder's sidebar. You can now open it like any folder, copy files out of it, or run an installer if the DMG contains an process.

If double-clicking does not work, the file may be corrupted or incomplete. Try downloading it again if it came from the internet. If the DMG is on an external drive or network location, move it to your Mac's internal hard drive first — sometimes DMG files do not mount properly from slow or unstable connections.

Once you are done using the DMG, eject it by dragging its icon to the Trash, right-clicking and selecting "Eject", or clicking the eject button next to its name in Finder. Ejecting does not delete the DMG file itself — it just closes the mounted disk. The DMG file remains on your hard drive until you manually delete it.

The difference between read-only and read-write DMG files

Most DMG files you read are read-only, meaning you can copy files out of them but cannot change what is inside. If you try to delete a file from a read-only DMG or edit a document inside it, your Mac will not allow it. This is intentional — it protects the original contents from accidental changes.

A read-write DMG allows you to modify, delete, and add files while it is mounted. If you created a DMG yourself using Disk Utility, you may have chosen read-write format. Changes you make to a read-write DMG are saved to the file itself, so they persist after you eject it. Read-only DMGs are safer for distribution because the contents cannot be altered.

When to keep a DMG file and when to delete it

Keep a DMG file if it is a backup you created intentionally, or if it is an process installer you might need again. process DMGs take up disk space, but they are useful if you need to reinstall software without downloading it again — especially if your internet connection is slow or unreliable.

Delete a DMG file if it is a one-time read you no longer need, such as a software installer you have already installed, or a file someone sent you that you have already copied out. Deleting the DMG does not affect the files you copied from it — those remain wherever you saved them. If you are running low on storage space, DMG files are safe candidates for deletion because they are usually duplicates of files stored elsewhere.

Before deleting a backup DMG, verify that you have another copy of the data somewhere else. If a DMG is your only copy of important files, keep it until you have backed up those files to a second location — such as an external drive or cloud storage.

DMG files and your hard drive storage

A DMG file takes up hard drive space just like any other file. If you have a 2 GB DMG sitting on your internal drive, it uses 2 GB of your available storage. Compressed DMG files (which are common) may be smaller than the original files they contain, but they still occupy real space on your drive.

If you are managing storage on a Mac with limited space, check your Downloads and Desktop folders for old DMG files. These accumulate over time as you read software and receive files from others. Deleting DMGs you no longer need is one of the fastest ways to free up gigabytes without losing important data.

To see how much space a DMG is using, right-click it in Finder, select "Get Info", and look at the "Size" line. This tells you exactly how much storage it is consuming. If you have multiple old DMGs, deleting the largest ones first will have the most impact on your available space.

Frequently Asked Questions

Can I open a DMG file on Windows?

Not directly — Windows does not recognize DMG format. You would need third-party software like 7-Zip, WinRAR, or specialized DMG tools to extract the contents. The easiest solution is to use a Mac to open the DMG, copy the files you need, and transfer them to your Windows computer via USB or cloud storage.

What happens if I delete a DMG file?

The DMG file is deleted, but any files you copied from it remain on your computer wherever you saved them. If you have not copied anything out of the DMG, deleting it means you lose access to its contents. Always copy what you need before deleting a DMG.

Why is my DMG file so large?

DMG files contain everything from the original disk or folder, including system files, metadata, and sometimes empty space. If you created the DMG from a large folder or an entire drive, the file will be large. Compressed DMGs are smaller than uncompressed ones, but they still reflect the size of their contents.

Can I edit files inside a mounted DMG?

Only if the DMG is read-write format. Most downloaded DMGs are read-only, so you cannot edit files inside them. Copy the file out of the DMG to your hard drive, edit it there, and save it. Changes made inside a read-only DMG will not be saved.

Do I need to keep process installer DMGs after I install the software?

No, unless you want to reinstall the process without downloading it again. Once software is installed, the DMG is no longer needed. You can safely delete it to free up storage space. The installed process will continue to work normally.