Easy2Boot is a Great MultiBoot USB utility
Easy2Boot (E2B) supports both USB Flash and USB HDD drives (FAT32, NTFS, exFAT, ext2/3/4)
You can prepare an Easy2Boot drive under Windows or linux. Once prepared, just copy over (or delete) any ISOs you like, at any time in the future.
No limit to the number of different Linux, Windows Install, WinPE, etc. ISOs or other payload files.http://www.easy2boot.com/
PS - I love Easy2Boot because it supports adding multiple windows images which other multiboot programs dose not. And it is very easy to add/update/remove ISO after creating MultiBoot USB.