When you create a guest virtual machine, most probably it expects to see a hard disk where it stores the data. VirtualBox needs to present real storage to this guest VM, and this guest VM sees the storage as a virtual hard disk.
There are three methods in which to achieve this:
- Most commonly, VirtualBox will use large image files on a real hard disk and present them to a guest as a virtual hard disk.
- Alternatively, if you have iSCSI storage servers, you can attach such a server to VirtualBox as well.
- Finally, as an advanced feature, you can allow a virtual machine to access one of your host disks directly.
Each such virtual storage device (image file, iSCSI target or physical hard disk) will need to be connected to the virtual hard disk controller that VirtualBox presents to a virtual machine. There are different disk controller standards, and VirtualBox supports most of the commonly used disk controllers