From VirtualBox manual: "Starting with version 1.4, as an alternative to using virtual disk images, VirtualBox can also present either entire physical hard disks or selected partitions thereof as virtual disks to virtual machines...
...this type of access is called "raw hard disk access"; it allows a guest OS to access its virtual hard disk without going through the host OS file system."
Do read the manual (part 9.7.1 at now) and do it at your own risk! Here we will talk only about permission problem you can get in Linux host. That, you will need read/write access for the entire disk (or selected partitions). For example:
$ VBoxManage internalcommands createrawvmdk -filename $HOME/.VirtualBox/HardDisks/sda2.vmdk -rawdisk /dev/sda -partitions 2
set up an image for access /dev/sda2 in Linux host. User needs read/write access to this partition. Otherwise, we will get some messages like this while adding sda2.vmdk to a virtual machine:
Check the physical disks properties:
$ ls -l /dev/sda*
brw-rw---- 1 root disk 8, 0 2010-08-30 21:53 /dev/sda
brw-rw---- 1 root disk 8, 1 2010-08-30 12:22 /dev/sda1
brw-rw---- 1 root disk 8, 2 2010-08-30 21:31 /dev/sda2
brw-rw---- 1 root disk 8, 5 2010-08-30 12:22 /dev/sda5
We can see one solution to grant permissions is adding username to the disk group, for example, by editing /etc/group file. And don't forget to re-login.