I bought for less than $50 at Radio Shack a 64gb flash drive. If you have a computer, plug both the flashdrive and a USB adaptor cable (assuming a Windows OS). Select "Turn on USB" on your Android device. On Windows computer, go to "My Computer" where you should see both drives. Copy/paste files back and forth as needed.
Without a computer: You can use your Android device and swap out 2 different SD cards. Download (from Google's Playstore) "File Manager" by Rhysoft (For tablets, "File Manager HD" is better). Create a folder in the phone's internal memory to temporarily 'hold' files to be backed up. Now, either 'unmount' sd card in 'Settings/SD Card Storage" and swap out cards, OR turn off device, remove SD Card, load new SD Card and boot up device. (Follow these steps! Just pulling SD Card out without unmounting it properly or device in off state could damage your files on the card.
One other way, what I do is this: I bought from Amazon a nice, powered by ac, 7 port USB hub, made by the Plugable company ( Plugable*|*USB Devices – Hubs, Cables, Graphics, Video, Docking, SATA, Keyboards, more
) - Also you will need a special "OTG" (On The Go) USB cable that has a male USB on one end (it can be found on Amazon for $2 or $4, must be an OTG cable!), and a micro male USB on the other that should fit into your device ( you may also need a female to female USB adaptor.
Using the USB hub with your fully charged device, AND "File Manager HD" (the regular "File Manager" app does not recognize External USB devices), you're able to move files between device and whatever is plugged into the 'hub'. (Using the hub also allows me to use a USB Logitech mouse/keyboard and joystick.)