XPS420 and 4TB drive with A07

Dell XPS 420.  One of the RAID 0 drives was starting to flake, so I purchased a 4TB drive that was listed as compatible with my system.

I've de-raided the machine, installed the drive, booted up to Windows 10...but can only use 1/2 the drive.  I updated the BIOS from 4 to 7.

I'm not sure what I'm missing, but I'm unable to find a way to: 

- switch the BIOS from Legacy mode

- switch the TB drive to Dynamic or GPT

In the end, I'd like to have the entire 4TB usable.  No worries about losing data - I'm all good with backups and such.  Just how do I get the bleeping compatible drive to work with my beloved 2008 Dell?

Thanks in advance!

Rick