m15 R4, storage bay question changing the drives

1T RAID 0 means two drives of 512G each, with the data striped across both.  If your intention is to clone/image the existing array to the new one, you will need an external drive to hold the image.  The RAID 0 array may need to be broken.

Depending on exactly which drives you get with the system, you may or may not be able to keep both of them in addition to the one you will be adding.  You can have two M.2 2280 drives, but a third drive must be 2230 (smaller form factor).  Since the WD you're installing is M.2 2280, if your system comes with two M.2 2230s (which is unlikely), you can keep both of those -- if it comes with two m.2 2280 drives, you can use only one of them.

Storage for this system:

• One M.2 2230/2280 solid-state drive
• One M.2 2230 solid-state drive and one 2280 solid-state drive
• Two M.2 2230 solid-state drives
• Two M.2 2280 solid-state drives
• One M.2 2230 solid-state drive and two M.2 2280 solid-state drives

Appreciate the help, guess because its in raid format I can't easily pull just 1 drive out and replace it with the 2TB? 

Before you do anything, thoroughly test the system for at least a few days to ensure everything is working correctly as shipped; if you replace the drive immediately, you may find yourself in for a world of pain and finger-pointing.

Once you're satisfied the system works correctly, see the system manual for setup details:

https://www.dell.com/support/home/en-us/product-support/product/alienware-m15-r4-laptop/docs

Got it, and is there a place I can refer to on how to break that array? I am new to that specific action.  Appreciate the help 

If your plan is to use the new drive as your boot device, unless there are now two smaller M.2 2230 drives, you will need to break that array in order to install the new drive.  Once that's done, you can set up one of the two RAID drives as a standalone and partition and format it.

If you do find you have two M.2 2230 drives, you will still need to remove both drives to install (and install Windows to) the new one, but you can then leave them as a RAID pair or you can set them up as separate 512G drives.

Hi @Spaznick  get external USB-C enclosure for M.2 2280 NVMe drive, and an ultrafast read/write 1TB or 2TB M.2 2280 PCIe Gen3 x4 NVMe drive. Use Macrium Reflect Free to create copy of RAID 0 (C:) boot drive onto the single M.2 2280 NVMe drive. You now have the original RAID 0 drives and the single NVMe drive, each capable of being your OS(C:) boot drive with all your original files intact. 

Your WD_Black SN850 is PCIe Gen4 x4 NVMe and will only work at PCIe Gen3 x4 read/write speeds in the m15 R4. 

Thanks so just got all this stuff setup, when I copy the raid c:, which option should I use in the program?  The copy windows bootable drive one?

then just swap out the drives and boot the computer?