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A single large expensive disk (SLED) is a data storage system that relies on one large disk, rather than an array of smaller disks. The term applies to an evolving philosophy in IT about how to handle data storage capacity for devices and pieces of hardware.
The term single large expensive disk is relative to the size of the disk, and the design of the storage media. It is often contrasted to a redundant array of independent disks (RAID) system, where more sophisticated systems involve replacing a single disk with a set of smaller or more agile disks or storage drives. RAID can help with efficiency, and also provides fault tolerance in many cases. In other words, if one disk in a RAID array fails, others can rebuild the data.
Another way to think about the term single large expensive disk is that basically, a SLED is just a traditional disk, such as the types of hard disk drives used in mainframes and computers up through the design of the personal computer in the 1980s. Up until recently, a SLED design was really the default design for many devices and networks. The diversification of data storage through RAID, along with the emergence of smaller and more capable hard drives for individual devices, has made SLED a kind of obsolescent strategy and a term that may be used mostly to refer to less efficient or suboptimal solutions.