A 1 TB NVMe drive in a video editing box comes up on a Monday morning with three numbers on screen, and no two of them tell the same story. The monitoring utility reports Percentage Used: 63, which a person reads as thirty-seven percent of the drive's life still in hand. A second utility reports 600 GB written, which reads as a drive that has hardly been touched. The drive's own log page reports Data Units Written: 1,171,875,000, which is not in any unit a person recognises at all. The warranty leaflet in the box says five years or 600 TB, whichever comes first. The drive was bought two and a half years ago.
Exactly one of those numbers is the one the warranty document is written against, and it is not the health percentage. What follows tracks that single counter from the NVMe field definition to the sentence in the warranty that decides the outcome. The same counter prints as 600 GB in one tool, 545.7 in another and 600 TB in a third, and the gap between the first and the last is a factor of 1,000.
One Counter, Three Readings
The number the drive actually keeps is Data Units Written, a field in the NVMe SMART and Health Information log page. Microsoft's Windows Driver documentation for the NVME_HEALTH_INFO_LOG structure defines it in two sentences that have to be read together. The first: “Indicates the number of 512 byte data units the host has written to the controller, not including metadata.” The second, and this is the one that gets dropped: “The value of this field is reported in thousands and is rounded up.”
So the field is not a count of 512-byte blocks. It is a count of thousands of them, and the multiplier from raw value to bytes is 512,000, not 512. Take the worked example. Multiply 1,171,875,000 by 1,000 to undo the thousands scaling and you have 1,171,875,000,000 units of 512 bytes. Multiply by 512 and you have 600,000,000,000,000 bytes.
Now the second reading. Sooner or later a tool or a forum calculator multiplies the raw value by 512 and stops. That gives 600,000,000,000 bytes, or 600 GB. It is not a rounding difference. It is low by a factor of 1,000, and on a drive at the end of its endurance rating it prints a figure that looks like a drive in its first month.
The third reading is subtler, and it is the one that costs people their claim. Convert 600,000,000,000,000 bytes using binary terabytes of 1,099,511,627,776 bytes each and you get 545.7. Print that under a label reading TB, as many storage viewers do, and the reader sees 545.7 against a 600 TB rating and concludes there are 54 TB of headroom left. A vendor footnote on byte counting says which terabyte these datasheets mean, though it marks the capacity row, not an endurance line. SanDisk's WD Green SATA SSD datasheet carries it as footnote 1: “1GB = one billion bytes and 1TB = one trillion bytes. Actual user capacity may be less depending on operating environment.”
One trillion bytes. So 600,000,000,000,000 bytes divided by 1,000,000,000,000 is 600 TB on the nose, and the binary reading understates the warranty-relevant total by 54.3 of the vendor's own terabytes, about nine percent. The bar reading 545.7 sits at 91 percent of the limit while the drive is at 100 percent of it.
What the Warranty Document Actually Names
Having the right total is only half of it. The other half is knowing which sentence the total is measured against, because the retail listing and the warranty document do not say the same thing. The listing says five years. The document says something narrower.
Samsung's SAMSUNG SSD Limited Warranty for the United States sets its warranty period, in the document's own capitals, as “THE SHORTER OF : (I) THE LIMITED WARRANTY PERIOD, WHICH DURATION IS SPECIFIED IN SECTION B OF THIS AGREEMENT, BEGINNING ON THE DATE THE PRODUCT WAS PURCHASED IN ITS ORIGINAL SEALED PACKAGING; OR (II) THE PERIOD ENDING ON THE DATE WHEN THE SSD HAS EXCEEDED ITS TBW (TOTAL BYTES WRITTEN) THRESHOLD AS MAY BE INDICATED BY SAMSUNG’S MAGICIAN SOFTWARE”. Read the whole clause and not either limb alone. Limb (I) on its own is the five-year promise from the retail page. Limb (II) on its own reads as though bytes were the only limit. The operative words are the two at the front, the shorter of, and the conjunction in the middle, plus the tail naming this warrantor's own utility as what may indicate the threshold.
The same structure appears at the other end of the market. The WD Green datasheet's footnote 5, attached to the warranty line, opens: “3 years or Max Endurance (TBW) limit, whichever occurs first.” Two limbs again, and again the byte limb is the one that goes unchecked.
There is a federal reason the byte limb has to be in the document you were handed rather than buried in a spec sheet. Under 16 CFR 701.3, the Federal Trade Commission's rule on written warranty terms, a warrantor of a consumer product costing more than fifteen dollars “shall clearly and conspicuously disclose in a single document in simple and readily understood language, the following items of information”, and item (4) of that list is “The point in time or event on which the warranty term commences, if different from the purchase date, and the time period or other measurement of warranty duration”. The phrase to notice in 16 CFR 701.3(a)(4) is other measurement. The rule anticipates that duration may be measured in something other than time. A terabytes-written threshold is exactly that other measurement, and the single document is where you should expect to find it.
Samsung's Section B table shows how far the limbs diverge inside one product line. An 850 PRO at 128 GB or 256 GB is listed at ten years or 150 TB, a 970 EVO at 1 TB at five years or 600 TB, an 860 QVO at 2 TB at five years or 1,200 TB. The longest headline period carries the smallest byte allowance, and the last two share a period while their allowances differ by a factor of two, so headline years cannot separate them at all for write-heavy work.
Why Percentage Used Drifts Away From Terabytes Written
If the byte counter is the one that matters, the obvious question is why the drive also reports a health percentage, and why it disagrees. It is measuring a different quantity, and the documentation says so outright. Percentage Used “Indicates a vendor specific estimate of the percentage of NVM subsystem life used, based on the actual usage and the manufacturer's prediction of NVM life.” A vendor specific estimate of NAND life. The definition does not name host bytes, and neither warranty document above names this field.
The field comes with further properties that rule it out as a warranty clock. First: “A value of 100 indicates that the estimated endurance of the NVM in the NVM subsystem has been consumed, but may not indicate an NVM subsystem failure. The value is allowed to exceed 100.” A drive can report 104 and be working correctly. Second: “This value is updated once per power-on hour (when the controller is not in a sleep state).” It is a once-an-hour estimate, not a running tally.
The bridge between the two quantities is write amplification. SanDisk's solution brief on use case impact on an SSD's lifespan defines it plainly: “WAF is the ratio of the number of actual writes to NAND divided by the host write requests.” The same brief says where the endurance rating comes from: “While an SSD's TBW spec is set by the vendor, it typically is a factor of the NAND type, expected WAF, and the SSD's capacity over a warranty period.” And it says how the number is measured: “TBW (terabytes written) values calculated using JEDEC client workload (JESD219) and vary by product capacity.”
Put those together and the divergence is arithmetic, not a defect. The rating is typically set against an expected amplification factor over a standard client workload, and your workload is not that workload. Suppose, purely to show the shape of it, that a 600 TB rating was set against an expected factor of 1.6, implying a NAND write budget of 960 TB. The brief publishes no figure, so treat 1.6 as an assumption.
Run sequential work at a real factor of 1.0 and 600 TB of host writes produce 600 TB of NAND writes, 63 percent of the assumed 960 TB budget. The drive reports a life estimate near 63 while the host-byte total has landed exactly on the warranty threshold. That is the Monday morning case. Now the opposite: small random writes at a real factor of 2.5, the pattern the brief warns against when it says “Host operating system and drivers should write in flash friendly manner such as large sequential blocks and avoid random small blocks or performing misaligned (i.e. crossing block boundaries) transfers.” Four hundred TB of host writes produce 1,000 TB of NAND writes, 104 percent of the assumed budget. The life estimate reads 104, past the nominal end of NAND life, while the host-byte total of 400 TB sits well inside the 600 TB limb and the warranty is open.
The two numbers can therefore point in opposite directions, and which way depends on how you write rather than on how sound the drive is. The brief's short version: “Use cases which have signficantly [sic] higher number of writes to the SSD can affect the long term lifespan of the SSD.” It is the same trap as a charger rating that is a ceiling rather than a promise, the problem behind the piece on why a 45 W label stops at 27 watts on a 9-volt device: one printed number stands in for a relationship, and the relationship governs.
Reading Your Own Drive in Four Steps
Every step here is a number you can check.
One. Pull the raw field, and also whatever utility your warranty names. On the NVMe log page the field is Data Units Written. Read the raw integer. If a utility offers only a tidy total in gigabytes, you cannot tell which of the three readings it used. On a SATA drive the nearest equivalent is SMART attribute 241, which the definition above does not cover, so the rest of this procedure is for NVMe.
Two. Convert with the 512,000 multiplier. Raw value times 1,000 times 512 gives bytes; divide by 1,000,000,000,000 for terabytes, because the vendor's footnote defines a terabyte as one trillion bytes. A figure a thousand times smaller than expected means the thousands scaling was dropped. A figure about nine percent smaller means binary units under a decimal label.
Three. Read both limbs of your own warranty document, not the retail page. Find the sentence that sets the period. It will name a date limb and a threshold limb joined by or, and it will say which governs, usually with the shorter of or whichever occurs first. Then find the threshold for your exact model and capacity, because it moves with capacity inside one product line.
Four. Work out which limb closes first at your write rate. Divide the threshold by your daily write average. On a five-year, 600 TB drive the break-even is 329 GB per day. Below that the date closes first and the threshold is irrelevant to you; above it, the threshold is your real warranty and the years on the box are decoration. The worked example drive averaged 682 GB per day and closed its byte limb at 2.41 years, with 21,144 power-on hours on the clock.
Two other fields answer a different question and are worth reading at the same time. Available Spare “Indicates a normalized percentage (0 to 100) of the remaining spare capacity available.” The reliability bit in the Critical Warning byte is set when “the NVM subsystem reliability has been degraded due to significant media related errors or any internal error that degrades NVM subsystem reliability”. Those tell you whether the drive is in trouble now; Data Units Written tells you whether the contract still covers you. Keeping the questions apart is the same discipline as reading a status code field by field, which is what checking the second hex digit first is about, and the reverse of the battery case, where the percentage genuinely is the contractual trigger, as in whether crossing 80 percent battery health entitles you to a free replacement.
When This Doesn't Apply
SATA drives do not have this log page. Attribute 241 is usually labelled Total LBAs Written, but no primary source for its units was consulted, so the 512,000 multiplier does not transfer. Use the vendor's own utility, which knows its own units, and treat third-party totals as unverified.
Some warranties have no byte limb. Where the document names a period and no threshold, there is nothing to convert and the date is the whole answer. Read before you calculate.
Reaching the threshold is not a failure. The documentation is explicit that a life estimate of 100 “may not indicate an NVM subsystem failure”, and a drive past its rating may run for years. The point is narrower: cover under this limited warranty has ended, and what happens at a service desk afterwards is a separate question.
Enterprise and endurance-graded drives are specified differently. Drive writes per day over a stated term is a common form, and the break-even arithmetic here assumes a single lifetime total.
The federal rule governs disclosure, not substance. It says what must appear in the document. It does not set the duration, and it does not displace state law on implied warranties.
What Would Invalidate This
A published expected write amplification factor. The 1.6 above is an assumption chosen to show the arithmetic; the brief states only that the rating is typically a factor of NAND type, expected write amplification and capacity. A published factor would replace the 960 TB budget and both percentage outcomes with real ones, and could reverse the direction of the divergence.
A vendor statement of what its life estimate counts. The field is documented as vendor specific. If a named vendor said its estimate derives from host bytes rather than NAND program cycles, then for that vendor the health percentage and the byte threshold would track each other and this divergence would collapse.
Claim adjudication practice. This follows warranty text, not what happens at a service desk. No primary source was found on whether any vendor has declined a claim on the byte limb alone, or whether that limb is checked routinely.
The printed wording of the model tables. The Samsung figures for the 850 PRO, 970 EVO and 860 QVO were read from the document as values rather than as the table's own typography, and the WD Green datasheet has no endurance row, so no figure is stated for that drive. If a threshold is printed in binary terabytes, the nine percent gap reverses in sign and every break-even moves with it.
A redraft of the disclosure rule. The argument that the threshold belongs in the document you were handed rests on the phrase other measurement in 16 CFR 701.3(a)(4). Narrowing duration to time alone would push the byte threshold back into the spec sheet, where few readers go.
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