I am running an HP xw8400 Workstation. I have already changed the drive from mbr2gpt... AND IT WORKED FINE. Now Inam wondering how to have my bios run this 3tb drive AS A 3TB. Essentially I want to change the Legacy settings to be uefi... which I have seen done on my machine, but BEFORE changing the drive.

I have working and fully max upgraded xw4600 , sister to yours.
MBR is limited to 2GB,(approx)
the OS must be 64bit, to go GPT,

are you saying to boot from this drive, full system not told
I run SSD as boot drive, 128GB. then drive 2 non boot can be any thing I want in w10 64bit, nobody uses w7, now, it dies, Q1, 2020.

UEFI safe boot needs, OS RELOADED, and to GPT.
and 64bit OS, it is NOT a BIOS issue. but do upgrade BIOS,
UEFI does not own GPT, you can run legacy with GPT, microsoft.com has full pages telling you how to do that.


I have w10 64bit, v1903 on mine. now. runs like champ.
once booted the OS runs the HDD not BIOS, a fact.
when you fresh load w10 64 from ISO the OS installer will set that 3TB as GPT , regardless of BIOS.
try it see.
but SSD drive there booting is always best. even on all my servers.HP and DELL.
you can set up w10 to put all data on drive, E: 3TB
or have 2 partitions there, programfiles and DATA.
then OS owns the SSD, alone.

hope that helps some.