U is a standard unit of measurement and a regular product of measure for assigning the peak in rack units and computer enclosures. An U equals 1.75 inches. For Example, there is a 4U chassis 7 inches high. There is a 40U rack case 70 inches high. It could also store 3 3U devices, with enough space left over for your miniature Star Wars action figure.
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The hard disk was invented on September 13, 1956 by IBM team led by Rey Johnson (considered as “father” of the disk drive). Earlier hard disk drives were large and cumbersome devices.