A good rule of thumb is that if it says somewhere "x inch" in a common kind of measurement, then the device never has any length of x inches.
Eaxmples:
- 3.5 in floppy disks have no 3.5 dimension (the disk is 86 mm, the cartridge is 94x90 mm, the drive is 100 mm).
- 3.5 in hard drives have neither 3.5 in platters nor are any of their external dimensions 3.5 in
- Ditto for 2.5 in drives, likely also true for 5.5 and 8 in drives.
- Any kind of inch-sensor size (e.g. 1/2, 2/3) is not the size of the sensor measured in any way.
- ...
5.25" disks are 5.25"/133mm along each side, making 5.25" a perfectly good description, in my view, for an object at this sort of scale. The disc inside must be 130mm in diameter, the case is presumably 3mm larger, and the whole thing is named after the size of the case. 5.25" = 133.35mm. I expect you could actually make a perfectly usable 5.25" disk that was exactly 5.25" wide...
Apparently 8" disks are 8"/203mm wide, no doubt for exactly the same reason, and with a rather similar result.
Eaxmples:
- 3.5 in floppy disks have no 3.5 dimension (the disk is 86 mm, the cartridge is 94x90 mm, the drive is 100 mm). - 3.5 in hard drives have neither 3.5 in platters nor are any of their external dimensions 3.5 in - Ditto for 2.5 in drives, likely also true for 5.5 and 8 in drives. - Any kind of inch-sensor size (e.g. 1/2, 2/3) is not the size of the sensor measured in any way. - ...