If you have a Win8 install ISO and you make a bootable FAT32 USB drive containing the files from the ISO, you can boot it in both MBR\CSM and UEFI mode.
The drive must be formatted as FAT32 in order to UEFI boot.
However, if the \sources\install.wim is over 4GB (as in the case of an AIO ISO), it cannot be placed on a FAT32 partition as the maximum file size for FAT32 is 4GB.
You can split the install.wim into 3GB-sized install.swm and install2.swm files using DISM or a Wim Splitting tool (as in WinToolKit). However, if you do this and boot from it (in MBr or UEFI mode) you will get a 'Windows cannot find the Microsoft Software License Terms. Make sure the installation sources are valid and restart the installation.' error just after entering in the 5x5 Product Key.
The solution is to add a \sources\ei.cfg file which suppresses the Product Key prompt and which also allows you to choose which version you want from the install.wim.
1. Make a single, Primary partition FAT32 USB drive in the normal way (e.g. RMPrepUSB, Rufus, Windows 7/8, etc.) with a bootmgr boot sector, etc. (or you can add grub4dos or whatever to make it MBR boot)
2. Extract the files from your Windows 8 source ISO to an empty folder on your system hard disk
3. Split the \sources\install.wim file into \sources\install.swm and \sources\install2.swm, etc, (WinToolKit - Wim Splitter is really easy to use for this!)
4. Delete the \sources\install.wim to leave the \sources\*.swm files
5. Add a \sources\ei.cfg file suitable for your install type (Volume licence or Retail/OEM)
6. Copy the entire folder contents to the USB drive
For a volume licence version of Win8/8.1 the \sources\EI.CFG should be
For a Retail\OEM version use
I have tested this on a OEM Win8.1 and it installed in both MBR\CSM mode and UEFI mode using Win8.1 OEM x64. If I delete the \sources\ei.cfg file, I get the error mentioned above.
This perhaps explains why some people have had success with this 'split wim' method on Win8 and some have not?
It also explains why MS say you cannot use split wims to install Win8/8.1 via Setup (but you can with Win 7). As shipped, Win 7 contains an ei.cfg file, Win 8 does not.
Q. How does Win8 know what version\SKU to install without an EI.cfg file?