You can add a 2.5" SATA SSD up to 2 TB in storage size. This from Crucial and guaranteed compatible. You may have been told that since Dell has not tested the system with a 2.5" SSD, but it will work.

https://www.crucial.com/usa/en/compatible-upgrade-for/Alienware/aurora-r8

@Cheetah08 

I also have an Aurora R8. I added a SATA SSD to my machine no problem. It slots into the 2.5" bay, make sure you buy a SATA III cable, plug it into the SSD, then motherboard connector, plug the power cable into it (already comes with your PC), and it's done. Then you'll need to make sure the system recognizes it in Windows 10. There was a format option I had to select -- sorry, forgot exactly what it was.

However... I found the speed is slower than other SSD solutions based on benchmarks. It's much slower than the NVMe SSD that comes with your Aurora by a huge factor. I also added an external Samsung SSD (via USB) and that's faster than the SATA SSD too. Seems SATA SSD is the slowest of the options. It's still better than a regular hard-drive though.

Thanks. I ended up getting the Samsung EVO 860 1TB SATA and SATA III Cable and sounds about 500-530mb/s should be about the speed for reading/writing. Plenty fast but will do some benchmark testing to ensure it works as advertised. 

@Cheetah08 

I returned my SATA SSD and went with a different approach. An NVMe PCI SSD plugged into a PCIe adapter, which then goes into the PCIe slot on the Aurora 8.

Wow does it make a huge difference. I'm now getting 3400MB/s read, and 3000MB/s write.  Easy to install too.

Here are the links:
https://www.amazon.com/gp/product/B07LGF54XR

https://www.amazon.com/gp/product/B07GFDVXVJ

Did you need to make any BIOS changes with this setup?

@Cheetah08 

If it helps, I have an Aurora R8 and just went through a period of adding drives.

I successfully added a SATA SSD to the 2.5" bay no problem (a SanDisk). However, it wasn't anywhere near as fast as the internal NVMe SSD so I took it out and returned it.

Instead, a much faster solution was this:

A PCI adapter that lets you plug an NVMe SSD into it. The PCI adapter then plugs into your PCI slot. This worked like a charm and I got blazing SSD speeds.  I went with:

*Adapter: RIITOP NVMe Adapter M.2 PCIe SSD to PCI-e
*SSD: Sabrent 1TB Rocket NVMe PCIe M.2 2280 Internal SSD

Also, before doing any of that I tried adding an external SSD via USB 3.1 . The speeds were decent, but Windows 10 kept losing its connection every now and then. I never figured out why.

I am waiting on a new Aurora R8 and want to add a second M.2 drive as you did. When you added the adapter and the Sabrent SSD, did your system add the drive without you having to make any BIOS changes? 

@Leehound 

It added the drive automatically on startup, no fuss no muss. Never had to touch BIOS. Though in Windows 10 I did have to go into "Manage" and format it for the first time. And then you want to optimize it (search for "Defragment & Optimize Drives" in Windows, then select the drive, then hit Optimize). That was it, then it just worked.

I am going to go your setup on my new rig. Thanks for being the guinea pig!

Did you or could you make that a boot drive?

Clean install with all other drives removed should have no issue.

Since the system must have a hard drive in order to format the SSD, when hard drive is  removed and windows is installed will the boot drive be C: or D:?

Is the EVO SSD still running smooth? Thinking about dropping one in my R8,

It is working fine, however I also installed an XPG SX8200 Pro M.2 drive which is somewhat faster and use this as my boot drive.

A PCI adapter that lets you plug an NVMe SSD into it. The PCI adapter then plugs into your PCI slot.

How many PCIe slots are on the Aurora R8 to allow for this?