Best SSD for a NAS: Cache, Pools and What Flash Really Costs

Start with the number that decides most of this page. Flash costs roughly 5 times what mechanical storage costs per terabyte at the cheapest end of each, which is why an SSD in a NAS is a targeted tool rather than a default. It earns its place on small-file and random workloads that mechanical drives handle badly, and it loses badly on the bulk capacity that most home arrays exist to hold.

Three picks below, then the cost arithmetic, then the part that saves people the most money: an honest look at whether a cache drive will do anything at all on your workload.

Best SSDs for a NAS by role

Beginner Smallest useful

Samsung

Samsung 870 EVO 1TB SATA SSD

$332.16

A known-good SATA SSD with a long track record in always-on machines. Check the price per terabyte before you buy this size, because the 2 TB is frequently better value.

Best for: A single cache or application volume rather than bulk storage.

Check price

Intermediate Best all-flash pick

Samsung

Samsung 870 EVO 2TB SATA SSD

$399.99

SATA caps at roughly 560 MB/s, which is still more than a 2.5GbE network can carry, so the interface is rarely the limit in a home NAS.

Best for: An all-flash array where silence and idle power matter more than raw speed.

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Expert NAS-rated endurance

Western Digital

WD Red SA500 2TB NAS SSD (M.2 2280)

$499.99

Rated for a much higher drive-writes-per-day figure than a desktop SSD, which is what caching duty actually demands.

Best for: A write cache that will be hammered daily. Overkill as a plain storage volume.

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Prices change often, confirm on Amazon. As an Amazon Associate we earn from qualifying purchases.

What does flash actually cost per terabyte?

Cost per terabyte is the only figure that matters for bulk storage, because a NAS serving large files is limited by the network long before it is limited by the drive. Here is every verified drive on this site, flash and mechanical together, on that single measure.

Cost per TB uses decimal terabytes as sold. Prices are the listing prices recorded at verification and change often, so confirm on Amazon. Mechanical and NAND pricing both ran well above their historical per-terabyte levels when these were checked, so treat the ratio as more durable than the absolute figures. The full mechanical ladder is on the cost per terabyte chart.

Now apply it to a real array, because per-drive prices understate the gap once parity is involved.

Usable capacity under RAID 5 or RAIDZ1, which is (n minus 1) times the smallest drive. Your NAS will report each figure about 9.05 percent smaller because it counts in binary tebibytes. Check any layout against the RAID capacity calculator.

Eight 2 TB SSDs cost more than four 16 TB mechanical drives and deliver less than a third of the capacity. That is the whole case against all-flash for a home archive, and no amount of sequential throughput changes it, because a mechanical array already saturates every network link most homes own.

Where does an SSD genuinely earn its place?

Flash wins on operations per second, not on throughput. A mechanical drive has to move a physical head, so it manages perhaps 100 to 200 random operations per second while an SSD manages tens of thousands. Every workload where flash transforms a NAS is a workload made of many small operations.

• Virtual machine images and containers. Random reads and writes across a large file, which is the worst case for a mechanical array and the best case for flash.
• Databases and mail stores. Same pattern, plus constant small writes.
• Photo catalogue browsing. Thumbnails and metadata are thousands of tiny reads, which is why a catalogue can feel slow on an array that streams 4K without effort.
• Active editing scratch space. A small fast pool for work in progress, with the finished output written back to the mechanical array.
• The system volume on a DIY build. A small SSD for the operating system keeps the array free to spin down and costs almost nothing.

Notice what is not on that list: streaming media, file sharing, backups and archives. Those are large sequential reads and writes, and a single 8 TB mechanical drive already sustains 150 to 260 MB per second, which is more than gigabit can carry. See hard drives against SSDs for a NAS for the full comparison.

Will an NVMe cache actually make my NAS faster?

A read cache stores recently or frequently read data on fast storage so a repeat read never touches the array. The word doing all the work in that sentence is repeat. A household that streams a different film every evening reads each file once, gets no cache hits at all, and sees no benefit from any amount of cache.

Three things are true at once, and vendors tend to mention only the first:

• Cache helps genuinely repetitive workloads a lot. A shared photo catalogue that four people browse, or a set of virtual machines with overlapping working sets, hits cache constantly.
• More memory often beats a cache drive. The operating system already caches metadata and hot blocks in RAM, and RAM is faster than any NVMe drive. Going from 4 GB to 16 GB with a 16 GB SODIMM is frequently the better hundred dollars.
• A write cache must be mirrored to be safe. Acknowledging a write before it reaches the array means an unmirrored cache drive failing at the wrong moment loses data the array never received. That means buying two drives, not one.

Endurance deserves a specific check for the write-cache role. Every write to the array passes through the cache, so annual write volume lands on that one drive. Against the ratings in the table above, an archive workload will never approach the limit and a busy write cache on a virtualisation host can consume it in a few years. Match the TBW figure to the job.

SATA or NVMe, and which slot?

The form factor decides the role more than the speed does. A 2.5 inch SATA SSD occupies a drive bay, which means it competes with a mechanical drive for the space that holds your capacity. An M.2 drive sits in a dedicated slot and costs you no bays at all.

That leads to the one genuine trap in this category. Many NAS models restrict their internal M.2 slots to cache duty and will not build a storage pool on them, and it is a firmware decision rather than a hardware limit, so two boxes with identical slots can behave differently. Check the specific model on its specification page before buying a large NVMe drive, because a 2 TB drive locked into a cache role is a poor use of $388.00.

A second trap: an M.2 slot is not automatically NVMe. The WD Red SA500 M.2 is an M.2 SATA drive, which fits the same physical slot on some machines and runs at SATA speed. Read the interface, not the shape.

Sequential figures are the manufacturer's published interface speeds. Note that on a NAS behind even a 10 gigabit link, which carries about 1,100 MB per second, the difference between a 560 MB/s SATA drive and a 7,000 MB/s NVMe drive is invisible for sequential transfers. It shows up only in random operations. Work the link ceilings through the network throughput calculator.

Do SSDs in a NAS need power loss protection?

Power loss protection is a bank of capacitors that lets a drive finish writing its internal buffer when the power disappears. Enterprise drives have it and consumer drives generally do not. On a NAS the honest answer is that it is a second line of defence, and the first line is the one to buy: a UPS that shuts the array down cleanly before the power actually goes.

Where it becomes worth caring about is a write cache on a busy array, because that drive is holding acknowledged writes the array has not yet received. If that is your configuration, mirror the cache and put the machine on a UPS regardless. Size the battery on the UPS runtime calculator.

Who should not buy the expert pick?

The WD Red SA500 2TB at $499.99 is the highest-endurance drive here at 1,300 TBW and is built for exactly the sustained-write cache role that consumer drives struggle with. If you are buying it as a read cache for a media library, you are buying the wrong thing. Media libraries do not repeat reads, the cache will sit cold, and the same money spent on a mechanical 8 TB drive adds capacity you will certainly use.

Buy flash when you can name the workload it is fixing. Buy memory before you buy cache. And buy capacity before either, because capacity is what home arrays actually run out of.

Related reading

• Hard drives against SSDs for a NAS for the head to head
• How to choose NAS drives for the mechanical side of the decision
• Best NAS hard drives for the bulk capacity tier
• Network throughput calculator to check whether speed is even your bottleneck
• Cost per terabyte chart for the full pricing picture

Deciding on a specific model? We review the Samsung 870 EVO 4TB SATA SSD review and Samsung 990 PRO 2TB NVMe SSD review in full.