As the virtual machine will most probably expect to see a hard disk built into its virtual computer, Oracle VM VirtualBox must be able to present real storage to the guest as a virtual hard disk. There are presently three methods by which to achieve this:
Oracle VM VirtualBox can use large image files on a real hard disk and present them to a guest as a virtual hard disk. This is the most common method, described in Section 5.2, “Disk Image Files (VDI, VMDK, VHD, HDD)”.
iSCSI storage servers can be attached to Oracle VM VirtualBox. This is described in Section 5.10, “iSCSI Servers”.
You can allow a virtual machine to access one of your host disks directly. This is an advanced feature, described in Using a Raw Host Hard Disk From a Guest.
Each such virtual storage device, such as an image file, iSCSI target, or physical hard disk, needs to be connected to the virtual hard disk controller that Oracle VM VirtualBox presents to a virtual machine. This is explained in the next section.