VirtualBox keeps track of all the hard disk, CD/DVD-ROM and floppy disk images which are in use by virtual machines. These are often referred to as "known media" and come from two sources:
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all media currently attached to virtual machines;
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"registered" media for compatibility with VirtualBox versions older than version 4.0. For details about how media registration has changed with version 4.0, please refer to Section 10.1, “Where VirtualBox stores its files”.
The known media can be viewed and changed in the Virtual Media Manager, which you can access from the "File" menu in the VirtualBox main window:
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The known media are conveniently grouped in three tabs for the three possible formats. These formats are:
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Hard disk images, either in VirtualBox's own Virtual Disk Image (VDI) format or in the third-party formats listed in the previous chapter;
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CD/DVD images in standard ISO format;
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floppy images in standard RAW format.
As you can see in the screenshot above, for each image, the Virtual Media 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.
The Virtual Media Manager allows you to
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remove an image from the registry (and optionally delete the image file when doing so);
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"release" an image, that is, detach it from a virtual machine if it is currently attached to one as a virtual hard disk.
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copy a virtual hard disk, to another one - target type can be different, options are - VDI, VHD or VMDK.
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modify the attributes of the disk image file - available options are : Normal, Immutable, Writethrough, Shareable, Multi-attach.
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refresh the values for the displayed attributes of the disk image currently selected in the window.
These commands are accessible once a medium has been selected either by selecting from the options shown at the top of the window, or by right-clicking the medium and selecting from the options shown on the drop-down menu.
Starting with version 4.0, to create new disk images, please use the "Storage" page in a virtual machine's settings dialog because disk images are now by default stored in each machine's own folder.
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 Section 5.6, “Cloning disk images” for instructions on this matter. Also, if you want to copy a virtual machine to another system, VirtualBox has an import/export facility that might be better suited for your needs; see Section 1.14, “Importing and exporting virtual machines”.