HDI files are Mac disk images, and you open them by double-clicking them in Finder

An HDI file is a compressed disk image — a single file that contains the contents of an entire disk or folder. On a Mac, double-clicking an HDI file automatically mounts it as a virtual disk that appears on your desktop and in Finder, just like a USB drive would. You do not need special software to open it. The Mac operating system handles this built-in.

When you double-click the HDI file, macOS reads it, verifies it is not corrupted, and then displays its contents. If the image is password-protected, you will be prompted to enter the password before it mounts. Once mounted, you can copy files from the disk image to your Mac, open documents inside it, or run installers directly from it.

If double-clicking does not work, or if you see an error message, the file may be incomplete, corrupted, or downloaded incorrectly. The sections below cover what to do in each situation.

Key Takeaways

  • Double-clicking an HDI file in Finder mounts it as a virtual disk on your Mac without needing any additional software.
  • A mounted HDI disk appears on your desktop and in the Finder sidebar, and you can copy files from it like any other drive.
  • If an HDI file will not open, check that the read finished completely and that the file size matches what the source listed.
  • You can eject a mounted HDI disk by dragging it to the Trash or right-clicking and selecting Eject, which closes the image without deleting the original file.
  • If you need to send an HDI file to someone else, the file itself is what you share — they will double-click it on their Mac to mount it.

The basic steps to mount an HDI file

Open Finder and navigate to the folder where your HDI file is stored. Locate the file — it will have a .hdi extension at the end of its name. Double-click it. macOS will begin reading the file and mounting it as a disk.

Within a few seconds to a minute (depending on the file size and your Mac's speed), the disk image will appear on your desktop as an icon that looks like a hard drive. It will also show up in the Finder sidebar under Locations. You can now open it like any folder and see the files inside. If the image is password-protected, a dialog box will appear asking you to enter the password before the disk mounts.

Once mounted, you can copy files from the disk image to your Mac by dragging them to another folder, or you can run installers or open documents directly from the mounted disk. The original HDI file remains unchanged — nothing you do inside the mounted disk modifies the HDI file itself.

What to do if the HDI file will not open

If double-clicking the HDI file produces no result or shows an error, the most common cause is an incomplete read. Check the file size by right-clicking the HDI file, selecting Get Info, and comparing the size shown to the size listed on the website where you downloaded it. If the sizes do not match, the read was interrupted. Delete the file and read it again, making sure the read completes fully before closing your browser or disconnecting from the internet.

If the file size matches but the file still will not open, the HDI file may be corrupted. Try downloading it again from the source. If you downloaded it from an email attachment, ask the sender to re-send it. If the file came from a website, try downloading it in a different browser — sometimes a browser cache can cause corruption during read.

Another possibility is that the file was created on an older version of macOS and is not compatible with your current system. This is rare, but if you are running a very new version of macOS and the HDI file is very old, you may need to contact the person or organization that created the file to ask whether they have an updated version.

How to eject a mounted HDI disk when you are done

When you finish using the files inside a mounted HDI disk, you should eject it. This closes the disk image and removes it from your desktop and Finder sidebar. Ejecting does not delete the original HDI file — it straightforward closes the virtual disk.

To eject, locate the mounted disk on your desktop or in the Finder sidebar, and drag it to the Trash. Alternatively, right-click the mounted disk and select Eject from the menu. The disk will disappear from your desktop when ready. The HDI file itself remains in the folder where you originally stored it, unchanged and ready to be mounted again whenever you need it.

Copying files from an HDI disk to your Mac permanently

If you want to keep files from inside the HDI disk on your Mac after you eject it, you must copy them out first. While the disk is mounted, open it in Finder, select the files you want to keep, and drag them to a folder on your Mac — such as Documents, Downloads, or Desktop. You can also right-click the files and select Copy, then navigate to the destination folder and paste them.

Once the files are copied to your Mac, you can eject the HDI disk safely. The files you copied will remain on your Mac. The original files inside the HDI image are not affected — they stay inside the image file itself. If you ever need those files again, you can mount the HDI disk again and access them.

Understanding why HDI files are used for storage and backup

HDI files are commonly used to distribute software, store backups, or share large collections of files because they compress the contents into a single file that is easier to read, email, or store on an external drive. Instead of sending someone fifty individual files, you can send one HDI file that contains all fifty. When they mount it on their Mac, they see all the files organized exactly as you arranged them.

HDI files can also be password-protected, which adds a layer of security if you are storing sensitive information. The person who created the HDI file can set a password, and anyone who wants to mount it will need to enter that password. This is useful for backups or shared files that contain personal or confidential information.

Because HDI files are disk images, they preserve the folder structure and file permissions from the original disk. This makes them useful for backing up your Mac or transferring a large project to another computer while keeping everything organized exactly as it was.

Frequently Asked Questions

Can I open an HDI file on a Windows computer?

No, HDI files are Mac-specific disk images and Windows does not recognize them natively. If you need to open an HDI file on Windows, you would need third-party software such as 7-Zip or other disk image utilities that support Mac formats. The easiest solution is to use a Mac to mount the HDI file, copy the files you need to a standard folder, and then transfer that folder to your Windows computer.

What is the difference between an HDI file and a DMG file?

Both are Mac disk images, but DMG is the more modern format and is used far more often today. HDI is an older format that you may encounter with legacy software or older backups. They work the same way — you double-click to mount them — but DMG files are more common in current Mac software distribution.

If I delete the HDI file, do I lose the files inside it?

Yes. The HDI file is the container for those files. If you delete the HDI file, the files inside it are gone. Before you delete an HDI file, make sure you have copied out any files you want to keep. Once the HDI file is in the Trash, you can recover it by opening the Trash and dragging it back to a folder, but once you empty the Trash, recovery is not possible through normal means.

Can I edit files inside a mounted HDI disk?

Yes, you can open and edit files inside a mounted HDI disk just as you would files on your Mac. However, the changes are saved inside the HDI file itself. If you want to keep those changes permanently on your Mac after you eject the disk, you should save the edited files to a folder on your Mac rather than leaving them inside the mounted image.

Why does my HDI file take a long time to mount?

Large HDI files can take several seconds or even a minute to mount, especially on older Macs or if your Mac's storage is nearly full. This is normal. If mounting takes longer than five minutes, the file may be corrupted or your Mac may not have enough free space. Check that you have at least a few gigabytes of free storage, and try restarting your Mac if the problem persists.