Without converting the Hard drive to an ISO or P2V format, is it possible to use a physical hard drive plugged in via USB as a Virtual Machine?
(Virtual Box is ideal, but not essential)
For VirtualBox, please refer to http://www.virtualbox.org/manual/ch09.html#rawdisk:
Note that this process involves creating a VMDK image file, but the file doesn't actually contain any data, but just the metadata referring to the disk or partition that you're pointing to.
Please also note that when you attempt to boot this, it will not be much different than moving the hard drive from one physical computer to another - one with different hardware (chipset / CPU / etc.). This will require your OS (you didn't mention what you're using) to be able to properly detect and install any appropriate drivers for the new virtual hardware that will be presented by VirtualBox. This may also cause similar difficulties if you attempt to use the drive natively again. Finally, be sure to heed the warning shown on this page: