Cant legacy boot any OS or installer

I have two Dell Inspiron 3250 that are a month old. They are both set to legacy boot and I can boot Win7, Win10, MSDART, etc without issue. I just ordered an identical system and setup the BIOS/UEFI exactly the same as the other systems. The new box crashes when I try to legacy boot anything including the Win 10 install DVD and bootable MSDART thumb drive and even the hard drive from one of the working systems (all work perfectly on the other 2 systems). I can boot/install under UEFI boot, but I want to use legacy. The only difference between the new system and the 2 old is the BIOS version (old is 2.4.2, new is 3.3.6)

I tried to downgrade the BIOS with the Windows utility and also a DOS bootable utility. Both methods report that the BIOS is locked.

I talked to Dell tech support and they informed me that the BIOS cant be upgrades/downgraded on that system which makes no sense to me. He also said that 3.3.6 doesnt support legace boot, which also makes no sense and legacy boot is clearly still an option that can be chosen

I also tried the RAM from one of the other computers just to be sure it wasn't an issue.
The BSOD I get when booting Win10 install DVD, or with the Working Win10 install from the other computer is PAGE_FAULT_IN_NONPAGED_AREA

Can anyone shed some light on what is happening here with legacy boot, and why I cant up/down grade BIOS.

Thanks

Check the motherboard in the new system. I suspect it has an upgraded motherboard and if it does, many new motherboards will not support anything other than Win 10. I have a new ASUS Z170 chipset motherboard (I just built a new system) and it ONLY supports Win 10. I had looked at an ASRock Z170 motherboard and it came with a special disc that would allow it to be Win 7/8.1 compatible.

The Dell Community (forum) would be a good place to post your question. Many user Dell experts there that help.
http://en.community.dell.com/

And you can create a UEFI bootable Windows Installer Flash drive without using the confusing and unneeded Rufus too....
Create a Bootable USB Flash Drive

Secure boot is disabled. When you enable legacy boot it gets disabled automatically.
As I said I am able to boot under legacy, just then no OS or setup runs, it crashes.

NavyLCDR - Idont want to create a UEFI boot, I want to install under legacy and can't.

It may be as fireberd stated, Motherboard looks identical but BIOS version is newer and maybe doesn't support legacy. Kinda strange since it is still an option. As far as only supporting Win10, I can't even run Win10 setup under legacy. Setup crashes as well with same error.


As suggested, I just posted in the Dell forums

THanks