wrote:
It shows up as Disk0. (the current 256 gb that came preinstalled as the boot drive is Disk1) And then it asked me if I want to choose a partition sytle, either MBR or GPT (with GPT being highlighted already on the dialog. I let it stay that way because MBR seems like something a user would choose if there is alreadfy a boot drive installed. Then finally I selected a drive letter. It wouldn't let me pick D, so I picked X.
As @BiggAl says, normally slot #28 would be used for an x16 graphics card. Assuming you can fit all rest of any other cards you need into the three motherboard expansion slots (and four backplane openings, the fourth of which is a "blank" opening), the second #26 "negotiable" x16 slot would normally be used for secondary cards of either x4 (like yours) or x8.
And ideally you would use the x1 slot for x1 cards, assuming there wasn't a double-wide x16 graphics card in slot #28 that obstructs the x1 slot from being used. I believe the #28 slot has a higher priority on the PCIe bus than #26. So if two or three of these slots were used for cards you should put the graphics card in #28 for best performance. Otherwise, it's not critical to use #26 or #28 for your M.2 adapter card if you don't also have an x16 graphics card to consider. Both will work.
As far as MBR vs. GPT, either will work for drives containing nothing but "data" partitions. But for creating a bootable Win10 partition on a drive, on a machine with UEFI BIOS you must use GPT for the drive that holds that partition. So if you ever do have another situation where you need to do first-time-use partitioning on a brand new raw drive, you should just always go with GPT which will always work to hold both Windows and "data" partitions on that drive. And GPT has additional advantages as well.
Aside from MBR vs. GPT and Win10/UEFI considerations, the real difference is that MBR can only support drives 2TB or smaller. MBR does not support a drive larger than 2TB.
Any drive of any size can use GPT. And specifically for drives larger than 2TB you MUST USE GPT.
Also, GPT supports any number of partitions, whereas MBR only supports four "primary" partitions (with one of those four also usable to sub-divide into multiple "logical" partitions).
Just go with GPT from now on, and keep your life simple. You will not have to adhere to any special considerations or restrictions.
As far as lettering it to be drive D vs. something else, and as to why you weren't offered D as a usable choice, my guess is that you have an optical CD/DVD drive installed in your machine and it already has drive letter D assigned to it. Hence why your new second M.2 drive can't be assigned D. But this isn't a true problem, since you can use DISKMGMT.MSC to change the drive letters of all your drives (other than C) to be whatever you'd like. You don't have to stick with X.
So, just run DISKMGMT, and first change your CD drive from D to something else (say "O", for "optical"). That will release D. then change your second M.2 drive from its current "X" back to "D" like you wanted to originally.