@lfb6 - Yes, some models it may work, but it's not very common is it? I've seen nothing but problems in many old systems right around UEFI intro era, that didn't have CSM or OS Type choice in BIOS, often had issues to boot UEFI
What brand and model is this so I can keep that BIOS in a proper folder, thanks. Also, for this thread, what chipset is that?

Thanks for the info, hopefully he can get it sorted out I suppose my comment above more applies to full UEFI Boot mode anyway, it may be OK like you say and graphics load out in legacy hopefully!

@Lost_N_BIOS Samsung NP300E5A (NP300E4A/ NP300E7A/ NP3430EA/ NP 3530EA), HM65 chipset.

I have another Samsung 400B4B (400B5B/200B4B/200B5B) notebook, also HM65, but AMI bios. I think it works the same way, but I haven't physical access right now. I'll attach the bios just in case...

Edit 240920: Just to confirm that the Samsung with AMI bios does boot in UEFI mode, too:

update:
Got the torx, unscrewed the backpanel, unplugged and removed the main battery. Located the CMOS battery and unlugged its supertiny connector to the motherboard. Hit the start button a couple of times to make sure there was no residual power keeping the cmos, replugged the jumper, battery and screed the backpanel back.
Low and behold, the laptop booted again.. :)
Entered the bios, and changed only the settings related to UEFI and rebooted with the original usb stick made with rufus. And this time windows detected the drive just fine, and installed without a hic.

I am now thinking that what lead me to problems, was flashing the bios, without a proper cmos reset after the flash. Reseting the CMOS on these ultrabooks is not for the fainthearted.