 If you install in Legacy and then change the BIOS to UEFI, the system will not boot. There is a way to do the conversion without data loss or reinstalling Windows 10 and this video should help.
Ok I installed a Samsung 970 EVO plus today on one of my PCI slots. had a little trouble seeing the drive at firast but was finally able to se it when I switched to legacy mode in BIOS.  I then installed Windows and rebooted.    I can get the computer to reboot in Legacy mode but not UFEI mode.    From what I understand I need to be in UFIE mode for some apps to work.    How can I enable booting in UFIE mode? I was going to try to add Boot location under UFEI in BIOS but am not certain of what I am doing.    Any help would be appreciated. 
Dell t5810 boot from NVME help