http://blogs.technet.com/inside_osd/archive/2007/09/21/how-do-i-assign-a-drive-letter-in-the-format-and-partition-disk-step.aspxThe short answer is, "you can't". Drive letters are only applicable to an instance of a running operating system. Any drive letters assigned in Windows PE would not be persisted when you reboot into the new operating system (or when you reboot to Windowsen-USCommunityServer 2.1 SP1 (Build: 61025.2)