Deepak, To boot into UEFI you need to boot from a FAT32 partition, and since USB Thumb drives only allow a single partition, the tools locks to allow FAT32 only. A secondary drive that is NTFS, or even a network drive could be used to store your WIM files if they are larger then the 4GB limitation with FAT32. Just boot up with the ImageAssist USB Key and point the corresponding tool to the NTFS drive, or use it to map a network drive.  Thanks, Gary
Hi Tyler I have tried with Commandline switch /secureboot but its not allowing me to change format partition to NTFS. Reply me at the earliest Thanks Deepak  
How to Create Secureboot DELL PE Drive with NTFS partition