VirtualBox is a program for running virtual machines. Basically, it runs another operating system in a virtual computer within your own computer. An ISO file is a file that contains a complete copy of the contents of a disc, typically a CD or DVD. You can access an ISO image with a virtual machine in VirtualBox as if it were a physical drive connected to a physical computer.
VirtualBox Mounts ISO Files
VirtualBox can connect to ISO files you have on your computer. This is useful for installing operating systems or other software or files on your virtual machines.
To have VirtualBox attach an ISO, select the virtual machine in the main menu in VirtualBox and click the "Settings" button. Click the "Storage" tab in the Settings menu. You should see an image of a disc and the word "Empty" under "Controller: IDE" in the Storage menu. Click "Empty" and select the CD/DVD icon.
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Click "Choose a virtual CD/DVD disk file" and navigate to the ISO file you want to connect to your virtual machine. The process is the same to have VirtualBox mount a CD or DVD ISO.
The virtual machine sees the drive as if it were a real disc drive. Depending on the operating system you're running, you may need to take additional steps to make the drive accessible to yourself or other users on the virtual machine, just as you would with a physical computer.
VirtualBox Mounts a CD
VirtualBox can also connect to a physical CD or DVD in your computer's drive. To do so, select the virtual machine in the main menu in VirtualBox and click the "Settings" button. Click the "Storage" tab in the Settings menu. You should see an image of a disc and the word "Empty" under "Controller: IDE" in the "Storage" menu. Click "Empty" and then click the CD/DVD icon.
In the pop-up menu, choose "Host Drive" followed by the identifier of the CD or DVD drive you want to connect to VirtualBox. Your virtual machine can then access the contents of the physical drive in your computer.
Other Disk Images
ISO files aren't the only virtual disk images supported by VirtualBox. The program typically stores virtual hard drive images, used to store data in virtual machines using its own VDI image format. It can also connect to other virtual drive images in formats commonly used by other virtualization programs from Microsoft and VMWare.