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.