Unless you really want to do a clean install of Windows (What version?) why not install both drives, with the original still connected to the blue SATA port on the motherboard.

Then boot from that original HDD and use Macrium Reflect (free) to image the old HDD onto the new one. When that's done disconnect the old HDD from the blue SATA port and plug the new HDD into that blue port. PC should boot from the new drive and you'll be done!*

And if you really want to speed this PC up, you could install an SSD in the M.2 slot on the motherboard as the boot drive, instead of using the 2T SATA HDD, which you could then use for storage.

*EDIT:  You can use Macrium to expand the size of the C: partition on the new HDD, after it's imaged to make use of all the extra space, assuming it's bigger than the original HDD.

I like fresh clean install on new drive. I am more experienced with install Win 10 via legacy BIOS than UEFI. So if I do the install for a client, I would go inside BIOS and change it to legacy mode, save changes, exit. Restart, Press F12 to bring up boot menu, choose boot from USB, proceed from there.

I would also choose AHCI in BIOS and save changes before install.

If I were to buy an SSD to put in the m.2, what type of SSD would be best suited for the board? And also, if I used the SSD to boot and kept the other two drives as data, would it be necessary to change the BIOS from RAID to AHCI? I read somewhere on these forums that you need RAID just to use multiple drives, but also read in the same thread that information was totally incorrect and could use AHCI. The replacement drive is the same size as the original (2TB). Depending how much the SSD will cost and ease of installation that's probably the route I'm going to take. Thanks for the quick responses!

Take out all drives including any m2 and remove cmos battery.

Put new blank drive in and use OEM System builder windows 10 dvd to clean install for that drive only with default bios settings.

https://www.neweggbusiness.com/Product/Product.aspx?Item=9B-32-350-238

 

AHCI is simpler to understand and implement for many users including me. RAID is when you want system to automatically create backup in case one storage device fails.

I would install os on the ssd either M.2 or sata without other hdd connected. This will make it easier to install because you can not pick the wrong drive. After install is all done and successful then connect the hdds as additional drives. Use disk management to partition and format the hdds.

You can choose either legacy bios mode or uefi to install os. I prefer legacy for simplicity.

newer bios and hardware remove legacy as an option.

32 BIT OS Support is also gone

UEFI mode requires a 64-bit operating system be installed (ex, Windows 10 Pro 64-bit, Windows 10 Home 64-bit, Ubuntu 16.04 SP1 LTS 64-bit).

Newer Dell systems unable to boot to internal boot device in Legacy boot mode

https://www.dell.com/support/article/en-us/sln309720

 

A lot of SSDs and cloning software don't play nice if RAID is enabled in BIOS. 

BUT- If you decide to clone the existing HDD onto another drive, HDD or SSD, rather than doing a clean install on the new drive, you'll need to make a simple change to Windows on the existing HDD before switching BIOS from RAID to AHCI. Otherwise, the PC won't boot from the old HDD again.

Once the PC boots from the new drive, cloned or clean installed, with no other drives connected, you probably will want to remove the old HDD and set it aside. You said it's slow and noisy which might mean it's on its way out. Will you still trust it for storage?  Besides, you'll have to initialize that old HDD (EVERYTHING gets deleted) to use it for storage, or the PC will continue to boot from the old HDD all the time, not from the new drive.

Crucial, and others, make M.2 SSDs that are compatible with the XPS 8900. I -personally- would go for a 500 GB SSD with 3400 MB/s Read, 3000 MB/s Write, to hold Windows and apps. And configure Win 10 and apps to store their default output files on the new 1T HDD, not on the SSD.

A 3400 MB/s Read M.2 SSD will be ~6 times faster than a SATA SSD, so an M.2 should give you better performance.

For the record what simple change would I need to make in Windows before changing from RAID to AHCI? At this point I'm planning on just cloning/imaging everything onto the new 2TB HDD, and upgrade to an SSD in the future. I was going to use Acronis True Image but am wondering if I'll have issues since the BIOS is set to RAID currently. Thanks again.

You should change BIOS to AHCI before you clone the drive, regardless of what imaging software you use.

1. Open Cmd, Run as administrator
2. Copy/paste this command, which will start Windows in Safe Mode the next time you reboot:
   bcdedit /set {current} safeboot minimal and press Enter
3. Close the Cmd window
4. Restart the computer and enter BIOS setup
5. Change the SATA mode from RAID to AHCI
6. Save the change and exit Setup. Windows will automatically boot in Safe Mode
7. Open Cmd again, as in step #1
8. Copy/paste this command, which will start Windows in Normal Mode the next time you reboot:
   bcdedit /deletevalue {current} safeboot and press Enter
9. Close the Cmd window
10. Reboot and Windows will automatically start with AHCI drivers enabled

EDITED