I'm not sure why, but Parallels is the only virtual desktop software that doesn't have built into there BIOS the ability to boot from a USB device. Many situations customers utilize the WinPE thumb drive to image machines or run utilities and I'm unable to replicate them because of this limitation and I really don't want to run two virtual desktop software packages on my MacBook Pro just for this important feature. I searched the forums and it appears this is a known issue since 2007 but still no solution from Parallels. Pretty disappointing.
