SD cards: the cheap way to boost laptop storage

24 Aug 2011
Apple SSD

An increasing number of laptops these days boast SSDs, but capacities are rising quite slowly. For some people, 128GB as your main drive might be enough, but if you want more, is it worth shelling out the huge fees charged by manufacturers to upgrade to a higher capacity SSD, or can you make do with alternative storage?

To find out, we ran our standard file transfer tests – first between a RAM disk and the SSD of a brand new laptop, then between a RAM disk and a variety of external storage devices.

The results are in the table at the bottom of this post.

The SSD

The tidiest upgrade is to a larger internal SSD, and there’s no doubt this is also best for performance. With a single 1.5GB file, the SSD in our test MacBook Air delivered read and write speeds of 187MB/sec and 156MB/sec. More importantly (you’ll see why later), with 1.5GB of tiny files its read and write speeds were a healthy 87MB/sec and 75MB/sec.

The big problem is the hefty price of a bigger SSD, with Apple charging £250 to step up from 128GB to 256GB in its 13in MacBook Air, and Sony charging £410 for the same upgrade in the VAIO Z. That’s a lot of money.

Sony SSD pricing

The external hard disk

The first alternative is an external hard disk, and it’s a cost-efficient way of adding storage, particularly for files you won’t always need to hand. The winner of this month's USB 3 hard disk Labs (issue 204, in shops now!) costs only £51 inc VAT for a 500GB drive.

In our tests with a single 1.5GB file, it achieved identical read and write speeds of 82MB/sec. With 1.5GB of tiny files this figure fell, but only to 60MB/sec read and 51MB/sec write; not as fast as an SSD, but significantly cheaper.

Of course, not all laptops have USB 3 ports – the MacBook Air being one such example. In our last USB 2 hard disk Labs, the winner achieved 32MB/sec read and 28MB/sec write speeds with a single 1.5GB file, and 26MB/sec and 12MB/sec with 1.5GB of tiny files.

The SD card

Adding external storage is cheap and fast, but if you prefer the convenience of having something you don’t have to carry around, you could make use of the SD card slot. Now, SD cards aren’t built for the kind of constant writing that you do on your main hard disk. They have a limited number of guaranteed write cycles before the card risks failing, so they’re best considered for storing files you don’t update often – a media collection, for example.

There are also several speed categories of SD cards. Look for a class rating on the packaging: this refers to its minimum non-fragmented sequential write speed. So, Class 2 will do at least 2MB/sec, and Class 10 at least 10MB/sec. To confuse matters, some manufacturers use “x” ratings that have minimum rates even higher than Class 10.

Sure enough, in the large file test a Class 10 card saw read and write speeds of 30MB/sec and 23MB/sec. For Class 6 this was 18MB/sec and 15MB/sec, while Class 4 saw 16MB/sec and 6MB/sec. You wouldn’t want to write 64GB of data regularly, but for a one-off the speeds are fine.

With small files those cards had healthy read speeds too, from 44MB/sec on Class 10 down to 20MB/sec with Class 4. But the big problem with using an SD card in this way is writing multiple small files: transferring 1.5GB of files to a Class 10 card pummelled the speed down to below 1MB/sec, and that fell even further with lower classes. If you’re going to regularly write a lot of small files, these cards are a terrible choice.

The value question

For data that will be written once and largely stay unchanged, however, does an SD card offer a value alternative to an SSD upgrade? At the kind of large capacities where it’s feasible, we found several 32GB Class 10 cards on sale for less than £40 inc VAT, and 64GB Class 10 cards at around £100. That’s for basic cards; those rated faster and with a higher number of guaranteed write cycles can cost up to several hundred pounds, so you can pick and choose to suit your needs.

You’ll need an SDXC slot for 64GB cards, and some slots don’t accept the card fully inside – on the MacBook Air it protrudes by 8mm, ripe for the snapping. But if your laptop meets the requirements, and if you’re after only a quick boost in capacity for non-critical files, the sheer convenience of being able to leave an SD card in there at all times makes it a great way to save money. And at lower capacities we really are talking pocket money.

(Click to enlarge)

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