Using SD Card as external storage for Macbook Pro retina 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)?
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.