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Hard Disk Drive Outer Sizes

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  • 20-09-2015 10:13am
    #1
    old_aussie
    Registered Users Posts: 3,410 ✭✭✭


    Why aren't 2.5" HDD's called 2.75" HDD's and 3.5" HDD's called 4" HDD's?

    What set the naming convention at 2.5" and 3.5"?


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    #2
    excollier
    Closed Accounts Posts: 1,576 ✭✭✭excollier


    Size of the spinning platters inside. Simple.


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    #3
    old_aussie
    Registered Users Posts: 3,410 ✭✭✭old_aussie


    Hi,

    Thought it might be the platter size.

    I got confused when measuring the HDD enclosure size in a new case.

    Measuring enclosure size for an adapter to mount a 2.5" SSD in a 3.5 enclosure and found the 3.5" enclosure to be closer to 4".

    Wasn't sure if the adapter bracket was going to fit correctly.

    Thanks


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    #4
    Sam Russell
    Moderators, Science, Health & Environment Moderators Posts: 19,711 Mod ✭✭✭✭Sam Russell


    Actually, it goes back to IBM.

    IBM invented the floppy drive to do IPL (initial program load) on the 360 mainframes (instead of punched card) when Adam was a little chap. These were 8 inch diameter but came in a 12 inch by 8 inch drive, and stored 128 k bytes. Shugart then brought out a mini floppy that was 5.25 inch diameter in a 6 by 8 inch drive, and these had a capacity of 360k (iirc), and later double sided versions. Hard drives were produced within this drive size with a capacity of 5 mega bytes. Half height versions were produced as well. These appeared about the time IBM produced the IBM PC, but the first IBM PCs did not use them. (About 1980).

    A 3.5 inch floppy was designed by HP that had a hard case and was designed to fit in a shirt top pocket, and had a capacity of 720 kb, with a HD version of 1.44 mb. These were in a drive size of 6 by 4 inches. Hard drives were designed using 3.5 inch platters and had capacities initially of 10 mb, 20 mb, upwards. (About 1987)

    Each time the drive was made smaller, it was done by halving the size of the drive (not the platter). Height was also reduced like this.

    The original 8 inch floppies were 8 inches square and were HUGE by modern standards - as were the mobile bricks phones at the time.


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