Why people say hard disk drive is magnetic disk when it (platter) is not magnetic?

If we read articles on hard disk drive, it usually mentions as following:

"A hard disk drive (HDD) is a non-volatile computer storage device containing magnetic disks or platters rotating at high speeds."

I have opened laptop hard disk drives and removed the platters. I tested the platters if they are magnetic. They are not. But there is a small curved magnet at one corner. But the platter itself is not magnetic. So, people say magnetic disk or magnetic platter?

7 Answers

  • Fulano
    Lv 7
    8 months ago

    It's the same thing with the magnetic tape in cassettes and VHS's. It's not going to stick to a magnet either.

    If it was magnetic enough to stick to a magnet you wouldn't be able to read it. The idea is to have as small a magnetic charge as possible that is still readable so you can have more points of data on the surface.

  • Sky
    Lv 7
    8 months ago

    It's not the platters themselves that are magnetic but is the coating on the platters that is magnetic.  I once had a hard drive with glass platters (best secure data destruction ever); glass is not magnetic, but the coating on it is.  Media articles talking about magnetic disks or platters in a hard drive are just simplifying the description to save space from an unnecessarily long description.

  • 9 months ago

    You would have to ask yourself why an electromagnetic head is used to read an write the data on a nonmagnetic surface. The answer is it can't and doesn't. The coating used on the disk as weak magnetic properties.

  • 9 months ago

    the coating on the disk is a magnetic oxide

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  • 9 months ago

    the aluminium alloy platter has a minutely thin layer of material that at a very finite distance can be magnetised in very tiny increments which can be detected by the very small heads which float above that tiny area. remember 1 platter drives could easily be 1 terrabytes which makes each BIT very very tiny in terms of thickness/width and volume of magnetic material, so thin in fact it just adds a bronze colouration to the disk it is lyered onto. Each bit can hold traces of previous values up to 9 writes previously. If you put a magnet anywhere near it the material would get a very severe shock and might become unusable again.

  • 9 months ago

    You are confusing a magnetic material with a hard disk that has microscopic zones of weak magnetic influence.

    Magnets are great masses of aligned dipoles of atoms of iron or other magnetic materials that form poles of North and South for attraction to steel or iron.

    Memory that is magnetic on computer platters are of a few microns in measurement, and there are billions of them on a platter's surface.  Each is so weak by itself.  But an induction coil in the flying head is just a tiny bit of distance from it.  As it passes over the magnetized region, a variation in the current of the coil occurs (a 1) or not (a 0), depending if there is a north (or a south) or nothing there on the platter surface.  That analog signal is strengthened thousands of times and digitized into a data stream.

    So you see, if each magnetic zone of the platter is very tiny and weak, and close to a neighbor so they cancel out, you cannot attract any iron or steel to a platter (at least in gross terms that your eye can see).

    But if you take tiny iron filings and pour them over a recorded platter, you might see in a microscope how some stick to zones and stripes of the surface.  That shows the recorded data.

    The larger magnet you see is a starting spot for alignment of records when the disk is first being formatted.

    Source(s): Using computers since 1962 when magnetic disks were as big as beer kegs and held a few thousand bits of data. Manufacturing engineer for an electronics company that was using dedicated computers in the 1970s and 80s. College computer classes instructor. Building PC boxes for decades.
  • Dick
    Lv 7
    9 months ago

    Normally the platter itself is glass and it is coated with a super smooth metal coating that can be written to by a powerful electromagnet. If you ever take a rotating hard drive apart be very careful because you can break the disk itself and it tends to fracture into very sharp/pointed pieces. Disks used to be metal years ago but today it's much easier to produce very smooth glass discs which reduces the manufacturing costs.

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