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I am getting a new Macbook Pro retina display with 256GB flash storage. The capacity may be a little bit too tight, but I should be able to manage it. I am considering to get a 128GB SD card as a secondary storage. I'd store files that I do not frequently use in the SD card. It would be less bulky than an external HDD.

I heard that different SD cards can perform very differently with the MBP, so was seeking some advice on which product has the best performance. Does anyone know what models of SD cards have the best read/write speed when used with a Macbook Pro (with or without retina)?

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It's hard to tell just what you're asking here. If you are asking whether anyone has used an SD card as semi-permanent external storage, the answer is clearly yes, such people exist. If you are asking them to identify themselves, that sounds like a first line in a dialog, but not a stand-alone question. If you're asking what product you should buy, that's off-topic for this site. I suspect you have a question you want answered, but as you've posted it, it's difficult to determine what information would answer your question. –  Daniel Lawson Jun 27 '12 at 21:17
    
I heard that different sd cards can perform very differently with MBP, so was seeking some advice on which product to buy for MBP. I did not know this kind of question was not suitable for this site. –  Ki Kim Jun 27 '12 at 21:39
    
Now that you put it that way, it's a question we can answer. –  Daniel Lawson Jun 27 '12 at 23:02
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@KiKim There's a small semantic difference between your question and what Daniel said would be off-topic. "Which product do I buy" is off-topic, while "what factors are important that I consider so I can make an informed purchasing decision myself" is on-topic, and I believe your question to be of the latter type. –  CajunLuke Jun 27 '12 at 23:27
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up vote 5 down vote accepted

SD cards are categorized in different classes according to their speed. Generally, SD cards perform even slower than a normal HDD drive which is why I do not recommend using them as a major storage option.

You should invest in the most expensive type of SD to have an acceptable I/O performance. E.g. an Ultra High Speed SD card (UHS) such as this one.

For $179 you get 128GB storage which is even more expensive than some SSDs models at that capacity. But the I/O throughput is much lower. The minimum guarantee is only 60MB/s for this particular model. SSDs in the same prize category offer up to 550MB/s of transfer speeds.

In sum, you're better off using an external SSD an connect it via USB 3.0 or Thunderbolt for best I/O throughput.

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It's good to know that SD card is incomparably slower than SSD. I should be looking for an external SSD. Thanks for the info. –  Ki Kim Jun 29 '12 at 0:42
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Or you could use an external HD which is cheaper than SSD and faster than SD. –  SSteve Aug 31 '12 at 23:02
    
@SSteve Ha! Of course :) No idea why I did not mention that... I've been using flash based memory only in my hardware for quite a while now. –  gentmatt Sep 20 '12 at 9:40
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It's true an SD is considerable slower then an external SSD via Thunderbolt or even USB 3.0. But, if you're just looking for a way to keep your internal SSD free of low speed data, like say an iTunes collection or iPhoto collection, then this idea works just fine.

You don't even have to go crazy with UHS cards. Most type 10 cards with do just fine and you can plug it in and forget about it.

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