UEFI requires x64 AND FAT32. FAT32 only supports up to 4GB file sizes. Image files (.WIMs) are normally well larger that this.
I came across a great blog post (Link Here). The guy updated the built-in LTIApply.swf to auto apply an SWM file if available. He also wrote a Powershell UI, which didn’t work for me. No matter, I pulled out the LTIApply.wsf he made, dropped it in my Media\Content\Deploy\Scripts folder (overwrite existing). And split my WIM using the imagex utility, which is builtin into Windows ADK.
imagex /split image.wim image.swm 4000
Deleted the old .WIM, and all was well.
LTIApply.wsf: https://panaconsulting.egnyte.com/h-s/20140812/hbobCYOdrw
/Brian G