UEFI is the new graphical bios that works alongside the GPT format that allows you to use all 4tb of your drive.
When I reformatted my drive in Feb, I had deleted win 10 off C and was expecting to just be able to install it back on the same partition, instead win 10 said it couldn't go on the drive as it wasn't in GPT. I didn't know what that was, I sat there for a while wondering how i get win 10 back on the drive - I had gone from a broken Win 10 to no Win 10 at all - and then looked it up on another computer. Think I read I needed to delete all the partitions but even with a blank drive, win 10 refused to go on. I restarted pc and for some reason it let me continue
I guess I had mine set to Auto in my bios as when I tried to install 10 it simply refused to do it on a MBR disc. I was same as you are, I didn't know about the difference and it was only the refusal that bought it to my attention. If the auto setting has said legacy I would still be using MBR now, but the Auto setting let Windows stop me installing it as an MBR disc and left me sitting there for about an hour after I deleted win 10 off the hdd. All it took in the end was a restart of pc, it must have loaded the right style bios (as Auto lets me use either) and even though I deleted all the extra partitions win 10 is supposed to create, it accepted just one partition as being fine and finally installed win 10, to much rejoicing
My install is different, I don't think I could recreate it if I tried. Instead of the 4 partitions it tries to create, I have one big 232gb partition which does everything. I expect there might be a hidden UEFI boot partition, I don't know since that would only be 100mb but I can't see it in disk management.
Glad you could swap, hate to buy a 4tb drive on an old pc and not have that option... wonder if mums pc is UEFI, I have never looked in its bios.