In 1961, IBM introduced the concept of removable memory via disk drive. The IBM 1311 Disk Storage Drive provided storage for about two million characters and its size is about the size of a washing machine. 1311 used IBM Disk Pack which is an interchangeable pack that contains six 14-inch-diameter disks in a four-inch stack that weighs about 10 pounds. It has the advantages of unlimited offline capacity, secure offline storage characteristics of tape units, and portability between separate computer systems.