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HDD heads read the surface of HDD, giving a signal to the controller:

  1. For a single signal, is it about one bit, one sector, or something else?
    1. If it is one bit, how many kinds of signals are there? My naive guess is four:
      Fully magnetized, Not magnetized, Partially magnetized (rare), Unreadable(rare)
    2. If it is not one bit, what are the possible causes of the signal?
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It is a magnetic media, so there are flux changes that are read by the head:

  • The signal is an analog wave that must be amplified, cleaned up, then converted to 0 or 1
  • The data stream is encoded (e.g. RLL\MFM, etc.) and contains ECC correction bits at the end of each sector (4k sectors are used on modern hard disks), which allows the decoder to detect and correct errors
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  • Thank you. If it is always one of 0 or 1, then which signal is regarded as a bad bit, or bad sector?
    – imida k
    Commented Jun 5, 2020 at 12:16
  • The magnetic flux that is read is not interpreted directly as 0 or a 1 bits. Rather the starting flux state (while reading in the gap) is considered a 0 bit, and then each flux reversal indicates a bit inversion. A bit clock synchronizes the sampling. The string of bits are shifted in a register, and then stored as bytes in controller memory. When the entire sector has been read, the ECC is used to verify (and correct) the data.
    – sawdust
    Commented Jun 5, 2020 at 22:21
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Data stored on a hard disk is in a binary state, either being demagnetized or magnetized. That equates to a single bit, being a 0 or a 1.

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