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Why do 3.5 inch floppy disks have sometimes three and others four holes on the backside? What's the difference, what do this holes indicate and what are these used for?

3.5inch floppy disk back with 4 holes 3.5inch floppy disk back with 3 holes

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The upper two (or one) notches at the sides, are to grab and clamp the disk against ejection - an addition Sony made to the 3.5 design on request by Apple, which had that already patented for the twiggy drive.

The lower two round holes are for precision alignment used during production to hold it exact in place. They may as well be used by some of the higher capacity drives, like LS-120/Superdisk.

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    Great answer. Thanks. It seems you're the master of floppies, considering my other questions you've answered. As I couldn't find it, do you know of any visual representation of the ejection mechanism that demonstrates this? A video? Some photos? It would be much appreciated.
    – Bob Ortiz
    Sep 29, 2022 at 20:11
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    Any references?
    – U. Windl
    Sep 30, 2022 at 6:51
  • The holes 'used for alignment' engage a feature in floppy drives that will not allow the disk to seat correctly if they are absent or misplaced.
    – Whit3rd
    Sep 30, 2022 at 7:18
  • I found a video related to that ejection mechanism here: youtu.be/69WWqwEeslY?t=80
    – Bob Ortiz
    Oct 3, 2022 at 7:43
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The diskette housing is pressed down onto alignment pins that engage two of those holes, which keeps the motor axis at the right position that the diskette is free to rotate in its housing (doesn't rub the sides) and the sliding door on the diskette engages the retraction finger.

A third hole is for the write-protect tab, while a fourth is indicative of high data density (a superfloppy holds 1.4 Mbytes, while 'standard' doublesided hold 720k to 800k bytes).

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    You are looking at the wrong holes. None of the encircled holes have to do with write protect tab or signaling high density. The write protect tab is the slide mechanism at the bottom of the image.
    – Justme
    Sep 28, 2022 at 10:49
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    The notches don't engage anything; the 'four holes' reference opens the question to the other features.
    – Whit3rd
    Sep 28, 2022 at 10:59
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    @Whit3rd that's not how frame challenges work. You're supposed to solve the real problem. Not insist that this is literally what you asked.
    – candied_orange
    Sep 28, 2022 at 22:19
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    It's so weird to be old enough to having been using floppies regularly at one time, yet young enough so that a floppy is "obviously" 1.44 MB as "standard".
    – vsz
    Sep 30, 2022 at 4:09

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