SanDisk Ultra 128GB: it barely promises anything, and beats all of it

Why test the card everybody already has?

Because almost nobody has actually measured it. The SanDisk Ultra is one of the best-selling microSD cards on earth, it is what ships in bundles and sits in drawers, and it is the default answer when somebody asks what card to buy. The bench here has been heavy on dash cam bundles, house brands and one outright counterfeit, and a plain mainstream card was the reference point that was missing. It joins CKBench, the running index of every card on the bench, alongside the Kioxia Exceria G2 128GB that sets the "good" line here, the Samsung EVO Plus 128GB that is the other long-owned card in the corpus, and the SanDisk High Endurance 128GB that sets the top of it.

What the box claims, and what the card did

Three of those rows need a note. The read row is not a miss. 96.7 MB/s is where the UHS-I bus runs out of road, not where this card does: every UHS-I card I have tested lands between 96 and 97 MB/s on this reader, including ones from three different brands. A 100 MB/s claim cannot be confirmed or denied on a bench that tops out just below it, so the row says where the measurement came from rather than pretending to a verdict.

The write row is the interesting absence. SanDisk publishes no write speed for the Ultra line at all. Not a vague one, not a footnoted one. Both generations of the official spec sheet say only "write speed lower" and stop there. That is worth sitting with: a card sold for phones and dash cams that declines to say how fast it records. It means U1's 10 MB/s floor is the only commitment about sustained writing the product actually makes, and 10 MB/s is the lowest bar any UHS card carries.

The A-class row cannot be settled on any USB card reader, mine included. A1's floors are defined under a test procedure that needs command queuing, which USB readers do not implement. My numbers clear the thresholds on paper, and I am still not counting them, because a result obtained outside the specified procedure is not comparable to the threshold in either direction. It is marked not testable, which never counts against a card.

How does a card miss V60 by a third of a megabyte per second?

Here is the number that decides a card's video class: the worst ten seconds. Not the average, not the peak. Take every ten-second stretch of a five-minute continuous write, find the slowest one, and that is the figure a V-class has to clear. The rule exists because a camera dropping frames for ten seconds has already ruined the shot, no matter how good the average looked.

This card's slowest ten seconds ran at 59.66 MB/s. The V60 floor is 60.00. It misses by 0.34 MB/s, which is half a percent, and so it holds V30 instead.

What makes that interesting rather than trivial is that every other way of describing the same run says V60. The steady-state average is 61.06 MB/s. The mean across all 300 one-second samples is 61.07. The median is 61.13. Its first thirty seconds averaged 61.13. On any test that reports an average, this card is a V60 card and nobody would question it.

I checked whether one bad moment was to blame. The slowest single second of the run was 48.23 MB/s, and it lines up with a file-roll: the moment the test closes one file and opens the next, which carries the run's longest blocked write at 220 milliseconds. That is a tempting explanation and it does not survive. Swap that one second for the run's median and the worst window only climbs to 59.87, still under the line. Across the whole run, 12 of 291 ten-second windows fall below 60 MB/s, in three separate stretches. This card is not a V60 that got unlucky. Its natural writing rate simply sits one to two percent above the V60 floor, close enough that ordinary variation crosses it over and over.

None of which is a failure, because the card never claimed a V class in the first place. There is no V mark printed on it. What it claimed was U1, a 10 MB/s floor, and it cleared that by six times. If you want the full explanation of why these marks are floors rather than speeds, that is its own article.

Does an eight-year-old card slow down as it fills?

This one does not, and I have unusually strong evidence for that because the capacity test writes the entire card. Most cards have a small fast zone, a pseudo-SLC cache, that absorbs the first few gigabytes at high speed before dropping to the real rate underneath. The drop shows up as a knee in the write curve. This card has no knee anywhere. It starts at 61.13 MB/s, it ends at 61.06, and there is nothing in between that looks like a cliff, because there is no fast zone to fall out of. It writes at its native speed from the first second.

The five-minute test wrote 18.3 GB. The capacity test that followed wrote 127.8 GB, all of it, and averaged 61.65 MB/s over 35 minutes and 39 seconds. That is a one percent difference across a write seven times longer, covering every block on the device. It also sweeps evenly: 60.0, 60.4 and 59.7 MB/s across the first, middle and last tenth of the card, a spread of 1.1%. There is no slow region hiding at the far end, which is the first place wear tends to show.

Then it read all 127.8 GB back and compared every byte. Zero mismatched bytes. Zero read errors. Across 120 files and nearly eight years of service, there is no measurable damage anywhere on this card. That does not predict how much life is left in it, and I want to be clear that it cannot. What it does say is that the common belief that microSD cards quietly rot in a drawer did not happen to this one.

Does the SanDisk Ultra run hot?

Hotter than it should for the work it is doing, yes, and it made no practical difference in this test. The card body reached 155.6°F in a 76°F room at the end of the five-minute write, a rise of 79°F over ambient. The rise is the number worth carrying, because that is what stacks on top of whatever the air around the card already is: the same card in a windscreen-mounted dash cam on a hot afternoon starts from a much higher baseline than a desk does.

Where it looks poor is efficiency. The Samsung EVO Plus rose 59°F in the same room while sustaining 30% more throughput. Measured as degrees of rise per MB/s delivered, this card sits sixth out of the eight comparable 128 GB cards on the bench. An older manufacturing process doing less work for more heat is exactly the shape you would expect from a 2018 part, and it is the only measurement here where the card's age visibly counts against it.

It cost nothing in this run. Nothing throttled, nothing stalled, the longest single pause in the whole five minutes was 220 milliseconds, and the card went straight on to write another 127.8 GB at the same rate afterwards.

How old is this card, and which version is it?

The card face prints no speed and no model number, and that is a real problem for reviewing it, because SanDisk has sold the Ultra 128GB at 80, 100, 120, 140 and 150 MB/s across the years, several of those behind the same face design. Reviewing it against the wrong number would either flatter it or fail it for a promise it never made.

So I asked the chip. Every SD card carries a CID register, a small block of read-only data with the manufacturer, a product code, a serial number and a manufacture date. Read on a Raspberry Pi with the card in a native SD slot, this one reports September 2018, which makes it seven years and eleven months old and the oldest card on the bench. That date settles the version: SanDisk's spec sheet for the generation shipping then rates 64GB to 512GB at 100 MB/s, and the 120 MB/s version arrived on a later sheet that also requires a proprietary mode faster than UHS-I normally allows. This card never negotiates that mode on a reader I know can deliver it, which is the physical confirmation that it is the older, slower part. If you want to read your own card's register, there is a decoder for that.

How the SanDisk Ultra 128GB scores

Should you buy the SanDisk Ultra 128GB?

Buy it for a phone, a tablet, a handheld or a 1080p dash cam, where it has between six and forty times the margin the job needs and the A1 rating keeps app data responsive. Do not buy it for 4K video work, where the margin gets thin and the V30 mark will lock you out of cameras that insist on V60 regardless of what the card can actually do. That is not a criticism so much as a description: this is a phone card, sold as a phone card, and it does that job with room to spare.

What surprised me is how little it promises and how comfortably it clears all of it. A 10 MB/s floor, no published write speed, no video class, and it turns in 61 MB/s flat across an entire 128 GB fill with nothing hidden in the last tenth of the card. It landed within five percent of the Kioxia that sets the "good" line here, while being seven years older and costing a fraction of most things on this list.

One honest caveat on all of it: this is a single used card of unknown history, so nothing here generalizes to every SanDisk Ultra on the shelf, and the version currently sold is a newer and faster part than the one I tested. What this review can tell you is what nearly eight years does to a good mainstream card, and the answer turned out to be: not much.

Disclosure: This is a personal card I bought at retail years ago and have used since; nobody supplied it and nobody was told it was being tested. The card linked below is not the one I tested. SanDisk has moved the Ultra through four generations since 2018, and what ships today is the SDSQUJQ, rated up to 195 MB/s and carrying a V10 mark this card does not print. I am linking it because it is what you can actually buy, not because the numbers above describe it. A unit of the current version is on its way to the bench, and this review will link the comparison when it is done. This post contains affiliate links. As an Amazon Associate I earn from qualifying purchases, at no extra cost to you. Affiliate links like these and my YouTube channel are what keep the site running.