The option to create a bootable USB drive was added in MistyPE version 2014-04-12. This feature should be used with caution due to the risk of data loss. Make sure that the correct target drive is selected and manually back up the contents of this target drive (and any other volumes contained on the same disk). Please note that I have only tested this script in Windows 7 (SP1) and Windows 8.1 - it should also work in Windows Vista but will not function in any Operating Systems predating Windows Vista (Windows 2000/XP/etc.).
This project has two options for creating a bootable USB drive -
Create USB (GPT UEFI)
...I recommend that you use the first. If the source files are compatible with UEFI based systems then a BIOS and UEFI bootable stick will be created. Option 2 will only work on UEFI systems. For more details about UEFI/BIOS and GPT/MBR partitioning see here
The steps for manually creating a bootable USB drive are included elsewhere in this documentation - click on the links at the bottom of this page.
The Create USB script has a number of options, including -
1] Select Drive - ensure that you select the mount point (drive letter) allocated to the USB drive. This drive will be repartitioned and reformatted so back up any important data.
2] File System - Selecting FAT32 will provide better compatibility and is required if booting on UEFI systems.
3] Boot Type - RAM Boot or Flat Boot (or both) can be selected. Creating a Flat Boot WinPE took between 15 to 45 minutes in tests depending on the file system selected for the target drive - FAT32 is significantly quicker. These tests were carried out using a minimal build on a USB 2.0 drive.
4] Copy Disk Contents? - this option will only backup the Volume selected. If this volume is on a disk with multiple partitions then backup the contents of all other partitions manually.
The create USB script will automate the steps listed in the DiskPart section here. One primary partition will be created spanning the whole disk - this will be NTFS or FAT32 formatted with volume label set as WinPE.
The following WinPE versions can be booted on BIOS and UEFI based systems -
WinPE 2.x (64-bit only)
WinPE 3.x (64-bit only)
32-bit WinPE 2.x/3.x can only be booted on BIOS based systems.
The following is a very simplified explanation of the stages involved when booting WinPE from a USB drive (NOTE - If the BIOS does not support booting from USB devices then you will have to boot from a CD/DVD as an alternative) -
The boot device is selected in the BIOS.
Partition table in the master boot record (MBR) is read and the active primary partition is identified (for most USB sticks there will only be one partition).
The volume boot record (aka partition boot sector, partition boot record) of the active primary partition identified in the previous step is loaded.
The volume boot record code loads BOOTMGR - the boot loader used to boot all versions of Windows since Windows Vista.
BOOTMGR reads and loads the boot configuration database (BCD). BOOTMGR has to be in the root of the active partition (e.g. C:\bootmgr) - the BCD store must be contained in the \boot subfolder (e.g. C:\boot\BCD). Note - this path is hardcoded.
The command-line DiskPart utility can be used to prepare a USB (or Hard Disk) drive to boot WinPE. The steps involved are detailed here
The Rufus (GUI based) utility can be used as an alternative - see here