VirtualBox keeps an internal registry of all available hard disk, CD/DVD-ROM and floppy disk images. This registry can be viewed and changed in the Virtual Disk Manager, which you can access from the "File" menu in the VirtualBox main window:
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The Disk Image Manager shows you all images that are currently registered with VirtualBox, conveniently grouped in three tabs for the three possible formats. These formats are:
Hard disk images, either in VirtualBox's own Virtual Disk Image (VDI) format or in the widely supported VMDK format;
CD/DVD images in standard ISO format;
floppy images in standard RAW format.
Starting with version 1.4, VirtualBox also supports the widely supported VMDK format. This means that if you have created virtual hard disks with another virtualization product that uses the VMDK format, you will not have to recreate these images with VirtualBox, but can continue to use them. See Sectioná5.4, “VMDK image files” for details.
As you can see in the screenshot above, for each image, the Virtual Disk Manager shows you the full path of the image file and other information, such as the virtual machine the image is currently attached to, if any. Also, as can be seen in the screenshot, if you have created snapshots for a virtual machine, additional "differencing" hard disk images may automatically be created; see Sectioná3.4.4, “Snapshots” for details.
The Virtual Disk Manager allows you to
create new hard disk images using the "New" button; this will bring up the "Create Disk Image" wizard already described in Sectioná3.2, “Creating a virtual machine”;
import existing VDI or VMDK files from your hard drive into VirtualBox using the "Add" button;
remove an image from the registry (and optionally delete the image file when doing so);
"release" an image, that is, detach it from a virtual machine if it is currently attached to one as a virtual hard disk.
We recommend that you maintain two special folders on your system for keeping images: one for hard disk image files (which can, in the case of dynamically expanding images, grow to considerable sizes), and one for ISO files (which were probably downloaded from the Internet).
Hard disk image files can be copied onto other host systems and imported into virtual machines there, although certain guest systems (notably Windows 2000 and XP) will require that the new virtual machine be set up in a similar way to the old one.
Note
Do not simply make copies of virtual disk images. If you import such a second copy into a virtual machine, VirtualBox will complain with an error, since VirtualBox assigns a unique identifier (UUID) to each disk image to make sure it is only used once. See Sectioná5.3, “Cloning disk images” for instructions on this matter.
Details about the different container formats supported by VirtualBox are described in Chapterá5, Virtual storage.