Hard-disk drives (HDDs) have been around since IBM conceived of the need for extra computer storage in 1956. A hard disk uses magnetised platters made of aluminium, ceramics or even glass to store data. These are typically rotated at 5,400 or 7,200 revolutions per minute for drives in home PCs. An arm that hovers just above the platters reads data from and writes data to the disk. SSDs, meanwhile, are built very much like the USB fl ash drives that have become popular over the last decade. There are no moving parts in an SSD, which helps it to access data signifi cantly faster. SSDs use a type of memory called NAND, which is non-volatile: instead of writing a magnetic pattern to a ceramic substrate, it stores data as an electrical signal that it retains even after the computer is switched off . Each SSD features a small processor called a controller, which performs the same role as the read/write arm of an HDD
