Wanted to access a few files on backup of a Virtual Box vdi harddrive. So after quite a bit of Googling and prowling forums I found the following few commands that helped me mount a VDI file on my physical host:

First and foremost you'll need to install QEMU to give us a few needed tools to achieve our goals here. For quick reference just use:

sudo apt-get install qemu-kvm qemu virt-manager virt-viewer libvirt-bin  

After that you'll need to enable ndb, which lets you use the NBD protocol to share the disk image on the network.

See Mounting a QEMU Image for those seeking more information about the protocol and how you might use it with QEMU

To use ndb it needs to be enabled on the system using:

modprobe nbd  

Now we'll be able to mount the virtual hard drive as a "physical" device that can be read from ( and written to :) ) by the system:

qemu-nbd -c /dev/nbd0 <vdi-file>  

For those looking to reduce the size of their VDI files, this is where you'll be able to run zerofree -v /dev/nbd0p1 which will clear up quite a bit of space (Macbook AIR's rejoice !)

After which you can simply mount the harddrive as you would any other device:

mount -o remount,ro /dev/nbd0p1  

And can be unmounted in the usual way as well:

umount /dev/nbd0p1  

When you are finished, remember to "disconnect" the hard drive to avoid data loss:

qemu-nbd -d /dev/nbd0