I know it's fashionable to hate on the Nifty Minidrive: "overpriced SD adapter!" "I can get an equivalent one for $2!". Truth is, it's an ingenious hack; a well executed, high-quality concept conceived by two local Manchester lads with a Kickstarter campaign. I've tried the cheap alternatives and they just don't compare.
Yesterday I was in Staples (aka Home Depot) for various reasons, and on a whim I picked up a Sandisk 128GB SDXC card to replace the 64GB one I was already using with Nifty. A quick swap, and now I have a grand total of 640 GB of space on my MBPr. That was just too easy!
(This said, I hope Apple hurries up with the MBPr refresh so I can buy a new laptop with 1TB disk. I was really disappointed by the lack of hardware announcements at WWDC; my current MBPr is now 4 years old but I'm not going to drop two grand on a new machine with chips from 2014...)