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Posted on August 24th, 2011 by David Bayon

SD cards: the cheap way to boost laptop storage

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 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.

SD cards

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.

Transfer speeds

(Click to enlarge)

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Posted in: Hardware, How To, Random, View from the Labs


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10 Responses to “ SD cards: the cheap way to boost laptop storage ”

  1. GW Says:
    August 24th, 2011 at 12:20 pm

    where does a 7200rpm 500gb internal hard drive sit in this speed table?

  2. Mat Bailie Says:
    August 24th, 2011 at 1:38 pm

    I think the article assumes the starting position of already having an SSD in the laptop. All the same, I’d be interested to see the comparison with replacing the SSD with a HDD…

  3. David Wright Says:
    August 24th, 2011 at 2:13 pm

    The MacBook isn’t a good place to start, when measuring USB speeds.

    My 2004 Athlon 64 can average 35MB/s with an external USB2.0 drive (120GB of files, ranging from 2-3MB up to 600MB each). My iMac, using the same external drive manages around 12MB/s.

    There is something very strange about the USB performance on Apple computers. More modern machines are better, but they still aren’t up to snuff, compared to PCs of even half a decade ago.

  4. Damian Says:
    August 24th, 2011 at 4:38 pm

    The trouble with buying SD cards, be they SD, SDHC or SDXC, is that there’s so MANY dodgy copies out there at this moment in time.

    P.s. The majority of the UK don’t use macs so maybe another oem would have been a bit more relevant.

  5. nichomach Says:
    August 25th, 2011 at 11:18 am

    What about hybrid drives? Seagate are supposed to have fixed the performance problems with their Momentus XTs?

  6. Paul Says:
    August 25th, 2011 at 5:49 pm

    Though personally I favour my laptop with it’s 2 x 500GB hard drives.
    SSD’s are still way over priced to be cost effective and if you use a reputable Co from the channel Isles, you can pick up a 32gb SDHC for under £20

  7. Fred Says:
    August 26th, 2011 at 5:35 pm

    Why does PC Pro (the clue should be in the title) use a mac to test this stuff out? Wouldn’t it be a courtesy to your readers to use a test bed that is relevant to them? Glad I stopped subscribing.

  8. B.W.Moore. Says:
    September 1st, 2011 at 8:22 am

    I asked PCPro some weeks ago ” Why can’t we used SD Cards, and USB Memory as RAM with our various Devices.
    Any progress on information as to “Why not ? ” please ?

  9. Arsalan Siddiqui Says:
    December 1st, 2011 at 4:56 am

    Its right that SD cards is the cheap way to boost laptop storage but i am not using it because Hard-disk i s better then a SD card. Recently i purchased a refurbished laptop in a very cheap rates online from “Electro Computer Warehouse”, its really awesome and working very well they also have laptops and computer Accessories in a cheap rate, check it may be it will be help full to any one.

  10. Freyberger Says:
    March 2nd, 2013 at 4:18 am

    Great breakdown on the SD card section


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