A 2015 is probably R1 (first release) and therefore SATA-only. 

You should be able to determine which model you have, which will dictate the type of SSD you can use:

https://www.dell.com/support/kbdoc/en-us/000124353/how-to-find-the-product-model-of-your-dell-computer

The later releases added NVMe support, but the initial Alienware 15 is SATA only.

Hi @GurusGuru  welcome to this user to user discussion forum. This is not Dell Support.

The Samsung SSD PM851 is a SATA drive. Your system board may not be designed to accept M.2 PCIe 3.0 x4 NVMe drives. 

Click on Product Support and allow your system to be automatically detected and its identity to appear in this webpage. The original is Alienware 15 and the next revision is Alienware 15 R2. The original is usually described as Alienware 15 R1 when looking for parts on the internet. 

Open the Documentation tab and then scroll down to the Storage details, to see your Dell supported options. 

If you have a 2.5" bay, there is the option to replace the HDD with a two M.2 SATA tray to accept M.2 SATA drives that are faster than the HDD, and will provide additional storage capacity. 

ejn63 and crimsom, thanks for the reply. It is the initial Alienware 15. Had already checked Alienware 15 Specifications in the documentation section. It simply states - Storage - SSD - Two M.2 drives. Specifications are not clear as is in the newer models' specifications.

Now need to know what is the maximum capacity the Alienware 15 can support. Will 1TB Sata SSD work or the limit is 512 GB

Are any of these 1TB SSDs compatible?

Western Digital WD Blue m.2 SATA SSD, 560MB/s R, 530MB/s W, 1TB
or
Samsung 860 EVO 1TB SATA M.2 (2280) Internal Solid State Drive (SSD) (MZ-N6E1T0)

Hi @GurusGuru  thank you for confirming that you have a 15 R1 laptop. 

There is no maximum capacity for your SATA 6 Gbps Drives, only what the market can supply. There are two M.2 2280 slots and the market can provide 2TB SATA Rev. 3.0 (6Gb/s) drive cards. If the HDD is replaced with a dual tray, additional 2TB SATA drive cards can be installed. The M.2 SATA cards are faster than HDD, so cloning the OS(C:) drive to the SATA card gives improved operating system performance on the laptop. The SATA drive specification should typically say "utilises the latest 3D TLC NAND technology while supporting a full security suite that includes AES 256-bit hardware encryption". Check the SATA specification, because some SATA drives do not support hardware encryption and you may want this feature. 

The two SATA drives in your post were not hyperlinked. When you visit a SATA card manufactures webpage for SATA drive recommendations, search on Alienware 15 R1. 

Samsung 860 EVO SATA SSD M.2 2280 2TB  

Western Digital Blue SATA SSD M.2 2280 2TB  

SSDs and SATA 3.0: Why We Need 6 Gbps  750MB/s 

Crucial MX500 1TB 3D NAND SATA3 M.2 (2280SS) Internal SSD (CT1000MX500SSD4)
up to Read 560MB/s, Write 510MB/s
Samsung 860 EVO SSD 2TB SATA3 M.2 Internal SSD with V-NAND Technology (MZ-N6E2T0BW)
up to Read 550MB/s, Write 520MB/s
ADATA SU800 1TB SATA3 M.2 2280 3D NAND Internal SSD (ASU800NS38-1TT-C)
up to Read 550MB/s, Write 540MB/s

Top 10 Best SATA3 M.2 SSD Speed Reviews of 2021  

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All of them except the PCIe and NVMe ones will work for you. I have 2 1Tb Crucial SSD's in the Slots and It works just fine.