If a MacBook Pro or Air owner wants to expand the storage of his or her laptop, the price can be quite high directly through Apple. Transcend has just unveiled a more reasonably priced solution called the JetDrive Lite that goes into the SD card slot, but it does it without sticking out like a standard SD card would.
This is not the first device designed to expand the storage of a Mac via the SD card slot (PNY's StorEdge is a similar option), but what makes this one stand out is the fact that it's offered in different sizes for a range of newer MacBooks. This is meant to allow it a snugger fit, which should make it look cleaner when plugged in.
Having it plug in without sticking out allows it to serve as a semi-permanent storage solution, but without the cost of adding an internal SSD or larger HDD. Not only should this end up looking better than a traditional SD card sticking out, but it could prevent damage to the card when the laptop is being toted around in a bag.
Transcend is promising fast data transfer speeds with a max write speed of 60 MB/s and 95 MB/s read speed. That should prove to be sufficient for day-to-day use for most users.
The cards are built specifically for different MacBook models. They come in 64 GB and 128 GB options, but not all models can use the larger card. Specifically, the MacBook Pro (Retina) 13-inch model only has a 64GB option. 13-inch Airs and 15-inch Pros also have size options available. The 11-in Air lacks an SD slot, so it obviously isn't compatible at all.
All of the different card options are available for sale right now. As of this writing, the full price of the 128 GB models are US$119.99 and the 64 GB options are $59.99. However, Amazon has them listed for $79.99 and $39.99 respectively.
The video below shows the cards in use.
Source: Transcend
These were based around interchangable micro sd cards plugged into a custom "sd" card that fit flush, and was also made of aluminum, so it matched your macbook.