How do you measure the size of a hard drive?

The physical size of a hard drive is different from the nominal size of a hard drive. Nominal is derived from the Word name, so the trade names for sizes of hard drives are 2.5", 3.5" and so on. If you were a hardware engineer you would measure the diameter of the disk that spins within the hard drive assembly. Because there is such a stark difference of one inch between the different trade sizes, it is easy to rule out by process of elimination the sizes that the hard drive is not, based on the total physical dimensions. Under no circumstances should you ever open up a hard drive to measure the size of the disk inside unless you possess the required skill, knowledge, & manual dexterity to do so successfully. If the hard drive that I hold in my hand has a width of 2.75", than it would be safe to assume that it is, in fact, a 2.5" hard drive because no other size disk would logically be expected to be used in that device. Always handle a hard drive by the edges, & never by the top & bottom for fear of caving in, even slightly, the top & bottom rendering it inoperable.