Mount the ISO and open admin CMD... Code: dism.exe /online /enable-feature /featurename:NetFX3 /Source:X:\sources\sxs where X is the letter of the drive. Enabled it offline in mounted WIM with no problems.
So it was my fault. Win 8 x64, I was able to use a 4GB USB drive. Win 8.1 x64 4GB is too small. Thanks James.
I'm not sure if this has been brought up before, but I am using the Windows 8 disc to go from 8.1 preview to save settings, personal files and apps. After this, I will upgrade to the RTM of 8.1. I am just upgrading from the preview back to Windows 8. Just thought I would give everyone a heads up so those that don't want to reinstall everything can do this!
It does work. this is the command Elevated Command prompt. Change source:X change the X to the drive where you have the ISO mounted or your install media drive letter. Dism.exe /online /enable-feature /featurename:NetFX3 /All /Source:X:\sources\sxs /LimitAccess Further Info can be found here support.microsoft.com/kb/2785188
I used a 4GB drive for 8.1 Pro, I think... *looks at USB drive sticking out of the PC* Yes, I put the ISO's contents on a 4GB drive and installed it without any problem!