A hard disk drive that fits into your pocket. Such devices plug into the computer via USB for easy connection and removal. They also derive their power directly from the USB port. See portable hard drive
and USB drive
Pocket Hard Disk
In 2004, this Seagate hard disk put 5GB of storage in your pocket and plugged into a USB port. Today's portable hard drives hold hundreds of gigabytes and, although larger, can still fit into a pocket.