Using SD Card as external storage for Macbook Pro retina

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.

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.