This used to be the default action in previous older versions of RMPrepUSB (used to be F11).

Ctrl+Shift+F11 - boot from USB drive in 'snapshot' mode.

F11 - boot from USB drive but now with full read/write access (but the host Windows OS will not be able to access the drive whilst QEMU is running).

RMPrepUSB v2.1.707 and later versions have two QEMU 'boot from USB drive' options: 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.

Virtualbox boot from usb drive