How to Choose NAS Drives: CMR, Workload and Cost per TB

The drives are the purchase. On a four-bay build they are roughly two thirds of the money, on an eight-bay build closer to four fifths, and unlike the enclosure they get replaced on a rolling schedule for as long as you own the array. Three published numbers decide the whole thing: the recording method (CMR, never SMR), the workload rating in terabytes per year, and the cost per terabyte. Everything else on the box is secondary. Cost per terabyte across the drives below spans $36.25 to $66.50, a difference of 83 percent for the same terabyte of storage.

What actually makes a drive a NAS drive?

A NAS drive is a hard drive whose firmware and mechanics are specified for continuous operation in a multi-drive chassis. That is the whole definition, and it comes down to four concrete differences from a desktop drive.

• Time limited error recovery. A desktop drive meeting a bad sector will retry for a minute or more, because in a single-drive machine that read is the only copy. A RAID controller interprets a drive that stops answering as a dead drive and drops it from the array. NAS firmware caps the retry window at around seven seconds and returns an error, letting parity supply the missing data instead. Western Digital calls this TLER, Seagate calls it ERC.
• A published duty cycle. NAS drives are rated for 24x7 operation. Desktop drives are typically rated for an eight hour working day.
• A published workload rating in terabytes read and written per year, covered in its own section below.
• Vibration tolerance, and on the higher tiers, active rotational vibration sensors.

Notice what is not on that list: raw speed. A NAS drive is not faster than a desktop drive of the same class. You are buying predictable behaviour under sustained load and in the presence of seven other spindles shaking the same chassis. Compare the two headline families directly in WD Red Plus against Seagate IronWolf.

Why does SMR break a RAID array?

Conventional magnetic recording (CMR) writes each track in its own lane with a guard band between tracks. Shingled magnetic recording (SMR) overlaps the tracks like roof shingles, fitting perhaps 20 percent more data on the same platter. Reads are unaffected. Writes are the problem: because the write head is wider than the read head, changing one shingled track corrupts the tracks laid over it, so the drive must read the whole band, modify it in cache and write the band back.

On a desktop that penalty hides. The drive absorbs bursts into a CMR staging area and reorganises later while the machine is idle. In a NAS there is no later. A rebuild is a sustained, hours-long stream of writes across the entire surface of the replacement drive, which fills the staging area within minutes and then leaves the drive doing read-modify-write on every incoming block. Reported throughput can fall from well over 100 MB per second to single digit megabytes per second, which turns a one day rebuild into a multi-day one. On some controller and firmware combinations the drive stalls long enough that the array gives up and marks the new drive failed, which is the failure mode people actually hit.

Never put an SMR drive in a parity array at any price. This is not a performance preference, it is a data availability problem: the window during which your array has no redundancy stretches from hours into days, and it can end with the rebuild aborting. If a listing does not state CMR explicitly, look up the exact model number on the manufacturer datasheet before buying. Every drive priced on this page is CMR.

The practical trap is that SMR is cheap and it shows up where you are least suspicious: in external USB enclosures, in low capacity desktop drives, and in the 2 TB to 6 TB range of consumer families. If you are buying externals to shuck, read drive shucking explained before you open anything, because the drive inside is the entire question.

What does the workload rating in TB per year actually tell me?

The workload rating is the manufacturer's stated annual limit on data read and written to the drive, in terabytes, above which the warranty assumptions no longer hold. It is the single best one-number test of whether something is genuinely a NAS drive, because desktop drives usually publish no workload rating at all, and drives that do publish one fall into two clean tiers.

• 180 TB per year is the consumer NAS tier. Seagate IronWolf, WD Red Plus and the Toshiba N300 all sit here.
• 550 TB per year is the professional and enterprise tier. IronWolf Pro, WD Red Pro and Seagate Exos sit here, and it comes bundled with a five year warranty rather than three.

Now the part nobody says out loud: 180 TB per year is a very large number for a home server. It is 493 GB every single day, forever. The table below puts realistic home workloads against it.

Figures are per drive where the workload is spread across members, and whole-array where a scrub reads every member. A parity scrub is the one routine job that can push a home array past a consumer rating, because it reads every sector of every drive. Monthly scrubs on a 48 TB array are 576 TB per year, which is why scrub-heavy setups belong on the professional tier.

So the honest reading is: for a media library, a photo archive, a backup target or a Plex server, the 180 TB per year tier is not a compromise, it is correct. Step up to the 550 TB tier when you scrub weekly, when the array records surveillance video around the clock, or when the extra two years of warranty is worth the money to you on its own terms.

What do RPM, cache and helium fill actually buy?

Spindle speed sets rotational latency and, together with areal density, sequential throughput. A 7200 rpm drive waits on average 4.17 ms for a sector to come round against 5.56 ms at 5400 rpm. That difference is real and it is also almost invisible behind a network.

Here is the number that ends the argument for most home builds: one modern 3.5 inch NAS drive sustains 150 to 260 MB per second sequential, and gigabit Ethernet tops out around 113 MB per second. A single drive of any speed already saturates a gigabit link with room to spare. Spindle speed only becomes visible when the network stops being the bottleneck, which means 2.5GbE at roughly 280 MB per second or 10GbE at roughly 1,100 MB per second. See 2.5GbE against 10GbE before you pay for either.

Where faster spindles genuinely earn their keep is rebuild time, because a resilver is a whole-surface sequential job that no network throttles, and every hour shaved is an hour the array spends unprotected. Against that, 7200 rpm drives run several degrees hotter, draw a watt or two more each, and are audibly louder at seek. One thing worth noticing in the table below: the marketing tiers do not map cleanly onto spindle speed. The Seagate IronWolf 8TB is a 7200 rpm drive in the consumer tier, while the WD Red Plus 8TB is a 5640 rpm class drive at a higher price.

Cache is DRAM on the drive's own controller board used to buffer reads and writes. Current NAS drives ship 64 MB to 512 MB. Treat it as a tiebreaker rather than a criterion: your NAS has gigabytes of system RAM doing the same job one level up, and on ZFS the ARC in main memory dwarfs anything on the drive. A jump from 256 MB to 512 MB is not worth paying for on its own.

Helium fill is more interesting. Sealing the platter chamber with helium instead of air cuts aerodynamic drag, because helium is roughly one seventh the density. That allows more platters in the same 3.5 inch envelope, which is how capacities past 12 TB exist at all, and it lowers both power draw and operating temperature. In these families the crossover to helium happens around 12 TB. It is not a feature you choose, it is a consequence of choosing a large drive, and it is a mild point in favour of larger capacities: a helium 12 TB drive can idle at less power than two air-filled 6 TB drives holding the same data.

Do I need rotational vibration sensors?

Rotational vibration sensors are accelerometers that let a drive detect chassis vibration and compensate its head position in real time. The problem they solve is specific to multi-bay enclosures: eight spindles and two fans in one sheet metal box produce enough vibration that a head can be nudged off track, which shows up as retries and a quiet collapse in throughput rather than as an error.

The rule of thumb worth remembering is bay count. In a two-bay or four-bay desktop enclosure with rubber-isolated trays, RV sensors are a nice extra. From roughly six bays upward, and in any build where the drives are screwed rigidly to a steel cage, they matter, and they are one of the real reasons the Pro tiers cost more. The table further down lists which of these models publish them.

If you are building into a tower rather than an appliance, the mounting matters as much as the sensor. Rubber grommets on the drive cage screws are a few dollars and they cut transmitted vibration measurably. Pick up anti-vibration drive mounting screws and a set of right-angle SATA cables at the same time, because cable strain against a side panel is the other thing that pulls trays out of alignment.

Which capacity is cheapest per terabyte?

This is the table the rest of the page exists to support. Cost per terabyte is sticker price divided by the decimal terabytes printed on the label, and it is the only fair way to compare drives that differ by a factor of six in capacity.

Sorted by cost per terabyte, cheapest first. Capacities are decimal TB as printed on the label; your NAS will report each figure about 9.05 percent smaller because it counts in binary tebibytes, so a 20 TB drive correctly shows as 18.19 TiB. See the TB against TiB chart for the conversion at every size. Specifications are from the published datasheet for each exact model number. Prices are live listing prices at verification and move constantly, so confirm before ordering.

Read the curve, not the rows. Cost per terabyte falls steeply from 4 TB, flattens hard across 8 TB to 16 TB at around $36 to $44, and then creeps back up at 20 TB and 24 TB where you are paying a premium for density. The Seagate IronWolf 4TB at $54.75 per terabyte costs 51 percent more per terabyte than the Seagate IronWolf 8TB at $36.25. Buying four 4 TB drives instead of four 8 TB drives saves $284 today and costs you half your capacity, in the same four bays, drawing the same power.

The sweet spot is 12 TB. The Seagate IronWolf Pro 12TB at $36.50 per terabyte sits within a rounding error of the cheapest terabyte on the board while carrying the 550 TB per year rating, the five year warranty and RV sensors, and it halves the number of spindles you need for a given capacity against 8 TB drives. If you want fewer, larger drives, the IronWolf Pro 16TB ties for the outright cheapest terabyte here. Below 8 TB nothing makes sense unless the enclosure is already full and you are matching an existing member.

What do four bays of each capacity actually cost?

Cost per terabyte of raw capacity is only half the story, because parity takes a whole drive or two off the top. Below is what four bays costs at each capacity, and what a terabyte of usable space works out to under single parity.

Four identical drives in one array. With identical drives, Synology SHR is exactly RAID 5 and SHR-2 is exactly RAID 6, so the columns are shared. Cost per usable TB uses the single parity figure. Run your own drive set, including mixed sizes, through the RAID capacity calculator, which is the only cross-vendor one: it puts SHR and SHR-2 next to RAIDZ1, RAIDZ2 and RAIDZ3 and the standard RAID levels on the same physical drives.

NAS drive picks by build size

Beginner Smallest sensible NAS drive

Seagate

Seagate IronWolf 4TB NAS HDD (ST4000VN008)

$219.00

CMR recording, rotational vibration sensing and a NAS-rated workload figure. Below 4 TB the price per terabyte stops making sense.

Best for: A two-bay mirror where the library is documents and photos rather than video.

Check price

Intermediate Best capacity per dollar

Western Digital

WD Red Plus 8TB NAS HDD (WD80EFPX)

$353.99

Red Plus is the CMR line, which matters because SMR drives can take days to rebuild an array. 5,640 rpm also runs cooler and quieter than the Pro tier.

Best for: The default four-bay build. Buy all four at once so they age together.

Check price

Expert High capacity

Western Digital

WD Red Pro 20TB NAS HDD (WD202KFGX)

$836.99

Rated for 24-bay chassis and a 550 TB per year workload. The catch is rebuild time: a 20 TB member takes well over a day to resilver.

Best for: Large arrays running double parity. Do not run 20 TB drives on single parity.

Check price

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

Should I buy all my drives at once, and from one seller?

Buy them at the same time, because matched capacities keep the array simple, but split the order across two retailers or two order dates. The reasoning is boring and it holds up.

Drives arriving in one shipment from one seller frequently come from one production batch: same week, same line, same materials lot, same firmware revision. Any defect that traces back to that batch will express itself at a similar number of power-on hours across every affected unit. Now layer on the fact that every drive in an array accumulates almost identical hours and almost identical thermal cycles for its entire life. The result is failures that cluster rather than scatter, and clustering is precisely what parity is bad at: a second drive going marginal during the multi-day rebuild of the first is the scenario that loses arrays.

Splitting the order costs nothing and decorrelates the batch. Two other habits do the same job:

• Burn in every drive before it holds data. Infant mortality is real, and a full surface write plus verify catches the drives that were going to die in their first weeks while you still have an empty array. Our drive burn-in testing guide has the procedure.
• Record the serial number and purchase date of each drive against its bay. When a drive reports errors two years from now you want to know which physical tray to pull and whether it is still inside a warranty window, without powering the array down to find out.

On the odds themselves: large-fleet annualised failure rates for modern 3.5 inch drives run about 1.0 to 1.6 percent per year, rising once drives pass roughly five years of service. Across four drives over five years that compounds into a meaningful probability that at least one fails, which is the entire argument for parity and, separately, for backups. Put your own drive count and horizon into the drive failure probability calculator. We do not publish per-model failure rates, because the public datasets that exist cover enterprise deployments in climate-controlled racks and do not transfer to a cupboard.

How much does the warranty matter?

The warranty length is a proxy for the manufacturer's own expectation of service life, and it tracks the workload rating exactly: three years alongside 180 TB per year, five years alongside 550 TB. It is worth reading it as information rather than as insurance. What a warranty returns is a drive, usually a factory-recertified one, and never the data or the days of rebuild time.

Two practical points. First, the warranty clock runs from manufacture in some cases and from purchase in others, so check the status on the manufacturer's own lookup page using the serial number when the drives arrive rather than years later. Second, the two extra years on a Pro tier drive genuinely lands during the period when annualised failure rates start climbing, which is the strongest argument for the Pro tier after the workload rating.

None of this replaces a backup. Parity survives a drive dying, and a warranty replaces the metal. Neither one does anything about a deleted folder, a ransomware run, a failed controller writing garbage, or a house fire, which is what the 3-2-1 backup rule is for.

Which drive should I actually buy?

Work down this list and stop at the first line that describes you.

• A four-bay NAS you want to fill once and forget. Four IronWolf Pro 12TB drives, $1,752 for 36 TB usable under single parity, at the cheap end of the cost per terabyte curve with a five year warranty.
• A first NAS on a budget, two or four bays. The WD Red Plus 8TB or the cheaper-per-terabyte Seagate IronWolf 8TB. Both are CMR, both are rated 180 TB per year, and 8 TB is the smallest capacity that is not actively bad value.
• An eight-bay build where drive count is the cost driver. IronWolf Pro 16TB at the joint-lowest cost per terabyte on this page, or the Seagate Exos 20TB if noise is not a consideration.
• Maximum capacity per bay, cost secondary. WD Red Pro 20TB or WD Red Pro 24TB, 512 MB cache, RV sensors, five years.
• Matching an existing small array. Buy the same capacity you already run, because on every layout except SHR the array truncates every member to the smallest drive and strands the rest.

Whatever you choose, price the drives before you price the box. An eight-bay enclosure with four drives in it is a four-bay NAS that cost twice as much, and the order you buy a home server in is the cheapest mistake to avoid.

Related reading

• Best NAS hard drives for the picks by build size with full specifications
• RAID levels explained for what parity does to the capacity you just bought
• Drive price per terabyte chart for the same table kept current
• HDD against SSD for a NAS for where flash makes sense and where it does not
• How to migrate to a bigger NAS when these drives run out of room