OK I'll make a little guide for restoring Win 8 in UEFI mode on the Ideadpad S400/5, hope it helps some folks. Make sure anything you want on your Ideapad is backed up!
1) Things you will need:
- An ISO disk image of Windows 8 (non-pro) x64.
- An external USB HDD or USB stick with 8GB+.
- Some basic knowledge of how to partition and format disks, which is beyond the scope of this guide.
2) Preparing the installation media:
You'll need to make a primary partition on your external USB storage device around 6GB+, format it as FAT32 and mark it active. The FAT32 filesystem is required for booting from the USB device in UEFI mode. Once this is done use 7-Zip or similar to extract all the files from the Win 8 ISO image onto this new partition. You might also want to copy the WIFI and LAN drivers onto here from Lenovo's website, I can't recall if Windows 8 has default drivers for them built in.
3) Make sure BIOS is setup correctly:
Restart your laptop and keep tapping FN+F2 until the BIOS settings come up. Check here that USB boot is enabled, UEFI boot is enabled and Secure boot is disabled (you can reenable it after Win 8 is installed). Save settings and exit. Keep tapping FN+F12 as your laptop restarts to bring up the boot menu.
4) Setup Windows
If you performed step 2 correctly you should now see your external USB device as a EFI boot option in the boot menu. Select it and Windows 8 setup should start. During Windows 8 install choose advanced drive options and (I recommend) delete all partitions on your Ideapad's internal hard drive, and then set Windows 8 to install in the now unallocated space. Windows 8 will automatically create the necessary tiny UEFI and system partitions it needs along with the normal large C: partition.
5) All Done
That's it! The product key should be automatically detected from the BIOS and Windows 8 installed already activated (minus Lenovo bloatware which you can get from their website if you want it). Remember to reenable secure boot in BIOS if you want it - extra security is never a bad thing with computers. Also you can keep your USB device around as a way to reinstall/repair Win 8 if you ever need it.