Unmount the dmg file named “Install macOS Sierra.dmg” (if it is mounted:) Go to the top menu of “Disk Utility” window and select “images” and then “Scan image for restore” Go to the “Install macOS Sierra.dmg” and click open. Now you are able to restore the dmg-file to your usb drive.

File TypeApple Disk Image

What is a DMG file?

A DMG file is a mountable disk image used to distribute software to the macOS operating system. The file is typically downloaded from the Internet by a Mac user, then double-clicked to install an application on the computer.

Today, software developers typically build and distribute applications for Mac users as DMG, .APP, or .PKG files. DMG files are especially common since their contents can be verified upon installation to ensure they have not been tampered with and they can also be compressed to reduce the size of the installer.

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When a Mac user downloads a DMG file, he can simply double-click it to open it with the DiskImageMounter utility that comes with macOS. When the DMG file is opened, it mounts a virtual disk on the desktop and in an Apple Finder window as if it was a CD or a USB flash drive that was just inserted into the computer.

In the Finder window, the Mac user can double-click the program icon, which is an APP file, to run the app. Or the Mac user can drag the program icon to the 'Applications' folder icon to move it to the 'Applications' directory in macOS.

After the software stored in the DMG file is installed, the DMG file is no longer needed. Therefore, DMG files can be deleted after installing the software.

While DMG files are Mac-specific and not intended for use in Windows, there are several Windows programs that can extract their contents or convert them to other formats. Some programs include 7-Zip, PeaZip, and DMG2IMG.

NOTE: The DMG format replaced the .IMG file format, which was previously used in Mac OS Classic.

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