GROUP TEST: Mi­croSD cards

Stor­age is cheap, but some­times it pays to choose wisely. Jim Martin ex­plains how to choose a mi­croSD card that’s right for your needs and rounds up your best op­tions

Tech Advisor - - Contents -

Just about ev­ery­thing with a mem­ory card slot takes mi­croSD th­ese days. From phones and tablets to action cam­eras, dash cams and drones: they all use mi­croSD for stor­age. Just any old card won’t do though, es­pe­cially if you’re try­ing to record HD or even 4K video. Over the fol­low­ing pages we ex­plain how to choose a mi­croSD card that’s right for your needs.

Buy­ing guide

First of all, it’s im­por­tant to un­der­stand mi­croSD stan­dards as well as all the mark­ings. There are dif­fer­ent types of mi­croSD card, even if they look iden­ti­cal.


The first is mi­croSDHC. The HC stands for High Ca­pac­ity is cov­ers sizes from 4- to 32GB. Above 32GB is mi­croSDXC (XC stands for eX­tended Ca­pac­ity), and the largest card you can cur­rently buy is 200GB. How­ever, the lat­est phones, in­clud­ing the HTC 10, claim to sup­port mi­croSD cards up to 2TB. Check your de­vice’s sup­port be­fore buy­ing a card: many dash cams don’t sup­port SDXC, so are lim­ited to 32GB.


There are three dif­fer­ent stan­dards for speed, and you may see more than one on a card. The orig­i­nal speed mark­ing was a num­ber in­side the let­ter C (mid­dle in the im­age, left). The num­ber de­noted the min­i­mum sus­tained write speed, so a ‘Class 6’ card would be ca­pa­ble of writ­ing at 6MB/s – six megabytes per sec­ond. That’s a se­quen­tial write speed, so it only ap­plies when writ­ing large amounts of data (such as

when record­ing video) in se­quen­tial mem­ory cells. It doesn’t ap­ply to ran­dom 4KB writes, which is the typ­i­cal use in a phone or tablet when small amounts of data are writ­ten to ran­dom lo­ca­tions.

Many de­vices which record HD video de­mand a Class 10 card, but most Class 10 mi­croSD cards are ca­pa­ble of much more than the min­i­mum of 10MB/s write speed.

This is where the UHS sys­tem comes in. It stands for Ul­tra High Speed, and uses a num­ber in the let­ter U to de­note the class. A UHS class 1 card writes at a min­i­mum of 10MB/s and a UHS class 3 card writes at a min­i­mum of 30MB/s.

You may also see UHS-I or UHS-II on the card. This tells you which tech­nol­ogy the card uses. The UHS-I ‘bus’ can op­er­ate at up to 104MB/s, while the UHS-II bus can trans­fer data at up to 312MB/s. This doesn’t mean the card will read and write at those speeds, only the max­i­mums.

In or­der to ben­e­fit from the higher speeds avail­able with a UHS-I or UHSII mi­croSD card, you’ll need a de­vice that’s com­pat­i­ble with this stan­dard. You can eas­ily spot a UHS-II card as it has a sec­ond row of pins un­der­neath the main set. All the cards on test here are ei­ther Class 10 or UHS-I.

There’s a new speed class sys­tem, called the Video Class. You’ll start see­ing this on cards soon, as a V with a num­ber be­side it. Like the orig­i­nal Class sys­tem, it de­notes the min­i­mum se­quen­tial write speed in MB/s and ranges from V6 up to V90.

The SD As­so­ci­a­tion rec­om­mends the fol­low­ing classes of card for record­ing at dif­fer­ent video res­o­lu­tions:

Tem­per­a­ture, X-ray and shock-proof

Mi­croSD cards are tiny and easy to lose, but in ad­di­tion to buy­ing a carry case for your collection, it’s im­por­tant to choose cards that will sur­vive trav­el­ling and any other fac­tors which threaten their op­er­a­tion.

Some man­u­fac­tur­ers state that their cards are wa­ter-proof and also X-ray proof. How­ever, th­ese are char­ac­ter­is­tics of pretty much all mi­croSD cards. Data isn’t stored mag­net­i­cally, so air­port scan­ners shouldn’t pose a prob­lem, and as long as you’re not try­ing to read or write data to the card in a non-water­proof card reader un­der­wa­ter, a mi­croSD card should shrug off get­ting wet.

Cards can also be rated to sur­vive in cer­tain tem­per­a­tures, say from -25 to 85ºC, be ‘shock’ proof and more. As long as you’re not pay­ing more for such cards, there’s no harm in buy­ing one of th­ese cards so that you can claim on the war­ranty if it fails be­cause it got too hot or was vi­brated too much. Quite how you would prove the cause of fail­ure is another mat­ter.

War­ranty, there­fore, is more im­por­tant than any of th­ese things: check not only the du­ra­tion but also the terms but also what it cov­ers.

Which cards to buy

Our rec­om­men­da­tion is to stick to the well­known brands, which will of­fer a war­ranty on their cards. Rep­utable com­pa­nies in­clude: Toshiba, Sam­sung, San­Disk, Lexar, Kingston and Ver­ba­tim, among oth­ers.

There are plenty of fakes and coun­ter­feit mi­croSD cards, so make sure you buy from a trusted seller. If you see a card on eBay that’s a lot cheaper than you ex­pect it to be, there’s prob­a­bly a rea­son. Be­fore you buy a card, check the max­i­mum ca­pac­ity of your de­vice. Some are lim­ited to 32GB as they are SDHC, not SDXC. It’s tempt­ing to get a 128GB card for un­der £30, but it won’t work if your de­vice can’t ac­cess it.

Drones/action cam­eras

If you’re buy­ing stor­age with the in­ten­tion of record­ing 4K video, go for a card that’s UHS-I Class 3 rated. Many also rec­om­mend the same if you’re record­ing 1080p, es­pe­cially at high frame rates.


It’s hard to buy us­ing spec­i­fi­ca­tions, as it’s the small-file trans­fer speeds, which make a dif­fer­ence here.

Dash cams

Many de­vices which record HD video de­mand a Class 10 card, but most Class 10 mi­croSD cards are ca­pa­ble of much more than the min­i­mum of 10MB/s write speed

The re­quire­ments are sim­i­lar to drones and action cam­eras, but you’ll tend to find dash cams are less de­mand­ing when it comes to write speeds. Most man­u­fac­tur­ers rec­om­mend a Class 10 card or bet­ter.

How we test

We use Crys­talDiskMark to test the read and write speeds of each card. This tests both the se­quen­tial speeds (read­ing and writ­ing large blocks of data) and small-file per­for­mance, us­ing 4KB reads and writes.

Tests are car­ried out on our In­tel Core i7-based test rig over USB 3.0. We use the full-size SD adap­tors which come with cards and a Lexar Professional USB 3.0 Dual-Slot Reader. If a card comes with its own USB 3.0 adap­tor, as with Lexar’s own card, we use that in­stead.

UHS Speed Class Bus Speed Speed Class

UHS-I UHS-II Min­i­mum se­quen­tial write speed Card im­age 90MB/s 60MB/s 30MB/s 10MB/s 6MB/s 4MB/s 2MB/s Speed Class UHS Speed Class Video Class Speed Cor­re­spond­ing video for­mat Speed Class The nec­es­sary speed varies by each record­ing/play­back de­vice...

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