Before we get into specifics about what makes the Ultra such a good microSD, it's worth noting that microSD cards don't actually have too many uses these days, and you should make sure you actually need one before buying one. (Duh, but it needs to be said.) Most high-end smartphones–Android, iPhone and even Windows Phone–only offer internal storage and no microSD slots. There are a few exceptions that do support microSD like the Samsung Galaxy S III, the Barnes & Noble Nook tablet, and cheaper Windows Phones. So if you're getting a new phone, don't just assume you need a big microSD card for it. Check! And if you do need one, this is the card to get–but I don't recommend buying a microSD card to use with an adaptor in a digital camera with a full-size SD slot. More on that in a sec.
So what makes a great microSD card? If you need the card for a phone or tablet, its 4K (kilobyte) random read and write speeds are very important. In most computer file systems, applications will write little bits of data here and there in small chunks, scattering it all around the storage device. 4K speeds is a good measure of this type of usage, so for use as a main storage point for apps, a high random read and write speed is key. Sequential writing speed, on the other hand, is important for shooting photos or video–you need sustained throughput for large amounts of data, not bursts of speed for teensy bits of it. The SanDisk Ultra averaged sequential writes of 12 MB/s and random writes of about 2 MB/s when I tested it (it's normal for sequential speeds to be way faster).
For some reason, the 4K random write speeds on some Class 10 microSD cards are absolute junk.
Why am I laying all of this out in detail? Because, for some reason, the 4K random write speeds on some Class 10 microSD cards are absolute junk. Their sequential write speeds may be great, but their random write speeds are terrible. Take a look at these benchmarks taken on an Android phone, and you'll see the 4K random write speeds on some cards drop from 1-2 MB/s to .005 megabytes per second according to some users' tests. You'd definitely notice a difference with one of those crappy cards in your smartphone.
How fast does a microSD card have to be, anyway?
First, the basics: microSD cards follow the same standards as full-size SD cards, meaning a Class 2 card has to maintain minimum speeds of 2 megabytes per second, a Class 6 card must maintain minimum speeds of 6 MB/s, and so on. The SanDisk Ultra is a Class 10 card, meaning it transfers data at a minimum of 10 MB/s (and can go a lot faster than that). It also, somewhat confusingly, carries the newer UHS (Ultra-High-Speed) Class 1 rating, which also certifies a 10 MB/s minimum transfer rate, but is further optimized for high-bandwidth uses like HD video.
Speaking of big video files, the most demanding requirement I've found for microSD is recording 4K video in the GoPro Hero 3, which requires a constant write speed of 5.6 megabytes per second. I benchmarked a SanDisk Ultra myself and got consistent write speeds of 10 – 15 MB/s–plenty fast for the GoPro.
Note that you should not get a microSD and use it in an adapter to replace a proper SD card. That's because micro cards are, overall, slower than full-size SD. The Class 10 micro Ultra's write speeds topped out around 15 MB/s, while the full-size SanDisk Extreme hit speeds up to 40 MB/s. Virtually all microSD cards come with adapters that fit into full-size SD slots–handy to copy off photos and videos if you only have a full-size card reader–but it doesn't make sense to buy a microSD card for a full-size slot when the faster SanDisk Extreme SD is basically the same price.
If you need micro, buy micro. If you don't need micro, buy full-size. Simple.
Why the Sandisk Ultra?
At about $30 for 32GB or $16 for 16GB, the Ultra costs around a buck a gig or less, and it writes data fast enough for even the most demanding tasks today. The SanDisk Ultra turned out to be an easy pick for three reasons.
- It's as fast (or faster!) than all the other popular cards I could find, and is actually cheaper than some slower Class 10 cards.
- SanDisk has a great pedigree with SD cards and overwhelmingly positive customer reviews.
- The SanDisk Ultra scores high marks for both sequential and random write speeds, which makes it good for all types of devices and uses.
The SanDisk Ultra will be good in whatever micro device you use it in.PCMag's Alex Colon awarded it four stars, and in tests on a variety of Android phones running the Antutu benchmarking app, he found that "the read speeds consistently reached 30 MB/s and slightly above for each test, while the write speeds averaged out to 13.5 MB/s." His only complaint was price–in 2011, the 64GB version cost $220. It's only $60 now.
Out of 2100 reviews on Amazon, the Ultra has a 4.5 star average (1500 of the reviews give it a full five stars). It's the most popular and best-rated on Amazon, but let's compare it to the runners up.
Kingston's 16GB Class 4 card costs only $11 and mostly has 5 star reviews. But it's also a model that has been around since 2009, and a 4 MB/s minimum transfer speed is a bit slow for shooting video. It's only around five bucks cheaper than the SanDisk Ultra. Pay that extra five bucks, and you'll get a card that's good for years.
Samsung has a Class 10 card of its own with a 4.5 star average from 240 Amazon reviews. Benchmarks from one Amazon post show similar speeds to the SanDisk Ultra, though its random writes are only about 1 MB/s. It's a damn good card. The problem? Costs $68 for a 32GB microSD. No, thanks.
Adata's Class 10 card is about $3 less than SanDisk's Ultra–$27 for 32GB–and it looks good on Amazon, with a 4.5 star average (though only from 60 reviews). But when I looked it up on Newegg, a disturbing number of reviewers said it up and died after a few months. Users wrote "Stopped working after ~8 months," "Bleh died after 7 months," "abruptly fails more and more frequently as time goes on." Based on the volume of complaints, it's hard to recommend it over the SanDisk at the same price.
Patriot's 32GB Class 10 Signature microSD card costs about $27, too, and it seems like a decent card. A few Amazon reviews say that they had trouble getting the card to work in their Android phones, writing "I originally bought this for my Samsung Galaxy S (the first one). If I rebooted the phone, it would usually lose everything on the card…I now have the Samsung Galaxy S III. It doesn't like the card either." But it doesn't look like a universal problem. Benchmarks by multiple users show write speeds right on the 10 MB/s line and reads around 18 MB/s, so it's definitely slower than the SanDisk Ultra and only a few bucks cheaper.
Even SanDisk makes some other cards that compete with the Ultra's popularity. There's the $22 32GB SanDisk Mobile Class 4 card, but I feel like this is another situation where it's worth spending an extra 10 bucks. One user's benchmarks show average write speeds of about 4 MB/s. Not bad, but you could triple that with the Ultra and be covered for a few more years.
In a November 2011 comparison, Tom's Hardware benchmarked 15 microSD cards and found that SanDisk's Class 4 Mobile Ultra came out on top in several categories. They wrote "Although it's advertised as a Class 4 card, SanDisk's Mobile Ultra microSDHC matches the performance of the Class 10 cards we tested, going so far as to win the sequential read speed metric." The newer Class 10 Ultra we recommend is even better, and is a pretty modest investment at $1 per gigabyte.
There's also the SanDisk Extreme Pro, which is faster than the Ultra–SanDisk claims up to 95 MB/s R/W speeds just like its full-size Extreme Pro–but it costs $50 for 16GB. And frankly, you're going to have trouble finding a microSD device that can take advantage of that speed.
Wrapping It Up
The SanDisk Ultra microSD is an easy buy because it's fast enough for a smartphone or a video camera, and you're not going to save much more than $10 shopping for a slower card. It comes with a full-size SD adapter, so don't worry about buying one separately. And if it ever dies on you, SanDisk will replace it. The Ultra has a lifetime warranty.
P.S. If you go above 32GB, remember that larger 64GB cards use a standard called SDXC, which some older devices don't support. Make sure to check for SDXC compatibility if you decide to buy a 64 gig card.