Files can be transferred to the PowerDrive Slim using the accompanying PowerDrive iOS app, and at the same time, it can charge your iPhone. You'll need to supply a Lightning cable for transferring files and for charging your iPhone from the PowerDrive Slim.
Size wise, the PowerDrive Slim is thicker than an iPhone 6 Plus, but not quite as wide or tall, so it fits easily enough in a pocket. We liked the portability, and we were impressed with the app, especially when it came to the auto backup feature for photos.
The 32GB PowerDrive Slim can be purchased from Amazon for $59.99. Mazzo also sells the device in higher capacities up to 128GB, which will be available for purchase in the near future.