The QEMU available in RMPrepUSB is much slower and is 32-bit only but can be used for testing grub4dos menus and will be almost as successful as VBox (just a lot slower!). The VMUB utility dismounts the USB drive before VBox runs and so allows VBox full unlocked access to the USB drive. Although it is possible to boot from USB without using VMUB, you will find that some things don't work correctly and in some cases don't even boot successfully. I highly recommend using VBox+VMUB for USB booting. Oracle VM Virtual Box can boot from a USB drive with full write access using DavidB's Virtual Machine USB Boot application (see below).
Note however that if it boots under a VM it does not guarantee that it will boot from every system for real! This is an easy way to test if your bootable USB drive is really bootable without having to reboot your Windows system and boot it from the USB drive instead.
You can make QEMU, VMware Server or Oracle VirtualBox boot from the host USB drive.
