Tak se nám chlapci v MS trošku pochlapili. WinPE už není nutný bootovat přímo do RAM, ale v souboru readme.rtf právě čtu, že:

Booting Windows PE directly from CD-ROM or USB flash drive
You can boot Windows PE directly from a CD-ROM or USB flash drive (UFD), not into memory (RAM). To do so, refer to step 4 in the “Walkthrough: Boot Windows PE from Hard Disk” topic, replacing the references to “partition=c:” with “partition=boot”. For CD-ROM, you must still create an .iso file and then burn the image onto a CD-ROM. For UFD, you can copy the source files directly to a UFD device.