Computers can and need to access large amount of data. As complexity of tasks which can be delivered by computers have increased so has their need to store large amount of data. This data has to be stored in disks which keep data even when the computers are not powered on. Due these needs computers may have one or more disks holding this data. Personal computers are not designed to have many disks, but server class computers have many disks attached to them to support large data needs. Over time the number of disks needed became more tens or hundreds. This led to a natural evolution of disk assembly which was independent of server computers themselves.

Disk storage is made accessible to computers primarily in three ways, direct attached disks, disks attached to the servers over fiber or specialized SCSI cables and disks attached to the server over network cables.