Requirements for using physical disk

I am a new user of Parallels Workstation 6.
I would like to try using a physical disk in a virtual Ubuntu install, but I just get an error saying that I don't have permissions to do this. The partitions with Linux are Ext4.

I have also tried with the trial version to use a physical Windows 7 SP1 install in a virtual machine, but I get an error there too - saying that the hard drive cannot be connected.

The documentation seems very sparse at the point of using physical disks.
Are there special requirements for using physical disks in VMs? Is it even possible to mount existing partitions (with an installed OS), or have I misunderstood the whole thing?