Jesper Berg 
Western Digital’s WD Blue SN550 is an entry-level PCI-Express Gen3/NVMe SSD that was released in late 2019. It has since been revised with lower-end NAND Flash memory and also superseded by both the WD Blue SN570 (2021) and SN580 (2023).
In other words, the WD Black SN770 (launched in 2022) is not a successor to the SN550. It is rather a midrange product that fills a gap between Western Digital’s entry-level WD Blue SSDs and its high-end SN850/850X SSDs. Importantly, the SN770 is also capable of utilizing the newer PCIe Gen4 interface.
What these drives have in common, however, is that they lack onboard DRAM. Instead, they use part of the host system’s RAM as a so-called HMB (Host Memory Buffer) to speed up transfers.
SN770 Vs. SN550: Specifications
This table compares the 2TB capacities of the SN770 and SN550. Full specifications for all available capacities in the respective series can be found at the bottom of this page.
Last update on 2024-07-26 / Affiliate links / Images from Amazon Product Advertising API
Since the SN770 and SN550 are from different generations and tiers, it is not surprising that the WD_Black SN770 is ahead of the older Western Digital SSD in every measurable way. It uses newer BiCS5 NAND memory chips compared to the SN550’s 96L BiCS4 – at least according to the original spec sheet. The SN550 NAND was allegedly downgraded to an unspecified lesser variety mid-cycle.
Regardless of the changes to the SN550, it is rated at about half of the maximum sequential read speeds compared to the SN770, and the differences in random IOPS performance are also substantial.
Both models are so-colled DRAM-less SSDs, meaning that there is no physical DRAM memory on the M.2 PCB. The result is typically (but not always) reduced performance compared to SSDs with DRAM. As of 2023, the WD_Black SN770 remains among the fastest affordable Gen4 SSDs in the DRAM-less category.
SN770 Vs. SN550: Performance
SSD specifications are focused on theoretical maximum performance data, which can and usually do differ from synthetic benchmarks and everyday performance in the real world. Note that our testing was done with a 1TB SN550 dating from before the NAND downgrade, and a 2TB SN770, which is marginally slower than the 1TB capacity.
AS SSD
AS SSD measures sequential performance using incompressible data, resulting in lower numbers than those found on the spec sheets. The SN770 is nevertheless around twice as fast as the SN550 in these results.
CrystalDiskMark
In CrystalDiskMark, the SN770’s lead is extended to 120% (read) and 146% (write) performance over the SN550.
The large difference in sequential transfer rates is evened out with random data, which is generally better at predicting overall real-world performance. 4K Q1T1 read and write performance is about 50% and 36% higher with the SN770 versus the SN550.
Latency/Response Time
Differences are also less pronounced in terms of latency (here in microseconds), with the WD Blue SN550 not being as far behind the more modern SSD.
SN770 Vs. SN550: Real-World Performance
PCMark 10’s app start-up times are still an overall win for the WD Black, but the web browser loading time differences are largely insignificant.
In the FF 14 standalone benchmark, the SN770 cuts loading times by 24% over the SN550.
Conclusion
As is often the case, real-world data moderates the differences in theoretical maximum performance a great deal. However, the WD Black SN770 is still much faster than the SN550 in a majority of the tests.
Read more:
- Comparison of the WD_Black SN770 Vs SN850X
- Comparison of the WD_Black SN850X Vs. SN850 (non-X)
- Full review of the SN770
- Full review of the SN550
- Updated listing of the best M.2 SSDs
- Updated listing of the fastest SSDs overall (incl. SATA)
SN770 Series: Full Specifications
| WD Black SN770 | 250GB | 500GB | 1TB | 2TB |
|---|---|---|---|---|
| Form Factor | M.2 2280 | M.2 2280 | M.2 2280 | M.2 2280 |
| Interface / Protocol | PCIe 4.0 x4 / NVMe 1.4 | PCIe 4.0 x4 / NVMe 1.4 | PCIe 4.0 x4 / NVMe 1.4 | PCIe 4.0 x4 / NVMe 1.4 |
| Controller | Proprietary | Proprietary | Proprietary | Proprietary |
| DRAM | No | No | No | No |
| Memory | 112-Layer TLC | 112-Layer TLC | 112-Layer TLC | 112-Layer TLC |
| Sequential Read | 4,000 MB/s | 5,000 MB/s | 5,150 MB/s | 5,150 MB/s |
| Sequential Write | 2,000 MB/s | 4,000 MB/s | 4,900 MB/s | 4,850 MB/s |
| Random Read | 240K IOPS | 460K IOPS | 740K IOPS | 650K IOPS |
| Random Write | 470K IOPS | 800K IOPS | 800K IOPS | 800K IOPS |
| Endurance | 200 TBW | 300 TBW | 600 TBW | 1,200 TBW |
| Warranty | 5-Years | 5-Years | 5-Years | 5-Years |
| MSRP | $59.00 | $79.00 | $129.00 | $269.00 |
SN550 Series: Full Specifications
| WD Blue SN550 | 250GB (WDS250G2B0C) | 500GB (WDS500G2B0C) | 1TB (WDS100T2B0C) |
|---|---|---|---|
| Form Factor | M.2 2280 | M.2 2280 | M.2 2280 |
| Interface/ Protocol | PCIe 3.0 x4/ NVMe 1.4 | PCIe 3.0 x4/ NVMe 1.4 | PCIe 3.0 x4/ NVMe 1.4 |
| Controller | WD Proprietary | WD Proprietary | WD Proprietary |
| DRAM Cache | N/A | N/A | N/A |
| Memory | SanDisk 96L TLC | SanDisk 96L TLC | SanDisk 96L TLC |
| Sequential Read | 2,400 MB/s | 2,400 MB/s | 2,400 MB/s |
| Sequential Write | 950 MB/s | 1,750 MB/s | 1,750 MB/s |
| Random Read | 170,000 IOPS | 300,000 IOPS | 410,000 IOPS |
| Random Write | 135,000 IOPS | 240,000 IOPS | 405,000 IOPS |
| Endurance | 150 TBW | 300 TBW | 600 TBW |
| Warranty | 5 Years | 5 Years | 5 Years |
