File creation error with Deployment and Imaging Environment

I'm following this step by step guide here:
http://technet.microsoft.com/en-us/library/hh825212.aspx

I've installed all the required software in the guide on my Windows 7 x86 machine. I'm using two USB keys, one 4GB and the other 8GB. I've tried copy pasting the commands and typing them by hand, both fail with the following error:

C:\winpe_amd64>MakeWinPEMedia /UFD C:\winpe_amd64 U:
WARNING, ALL DATA ON DISK DRIVE B: WILL BE LOST!
Proceed with Format [Y,N]?Y
Formatting U:...
Setting the boot code on U:...
Copying files to U:...
File creation error - The parameter is incorrect.
Unable to create directory - U:\
ERROR: Failed to copy files to U:\.
C:\winpe_amd64>

I want to create a PE environment for UEFI (FAT32).