Back in the early days of hard disks, you had two choices, SLED or RAID. SLED systems used a single high capacity hard drive to store lots of data. The drive may have contained a platter that was 3 feet (1 meter) across. It was very expensive. RAID allowed users to link a bunch of smaller, cheaper hard drives together for redundancy and to save the cost of a SLED. As costs and physical sizes of high capacity hard drives decreased, the term SLED began to lose much of its meaning.