3 ms·

A gas provides a "bearing" for the heads to ride on as the disk spins below it.

rob74 2y ago
Right you are, forgot about that completely! IIRC this is important to prevent a headcrash if the drive is jolted while in operation (of course within certain limits).
- Brian_K_White 2y ago
  It's required to work at all, not just as some kind of guard rail. It's like the oil in a normal bearing, the oil isn't just to prevent a crankshaft crash in case the car is jolted while in operation, the oil is a part of the way the thing functions at all in the first place.
ahartmetz 2y ago
Maybe magnetism could work. HDD platters take very strong fields to change magnetization these days, so erasing the data with the positioning magnet(s) seems avoidable.
- amelius 2y ago
  So write too many zeros to your drive, the disk becomes demagnetized, and the head will crash?
  - Faaak 2y ago
    With another encoding like Manchester, it wouldn't;-)
  - frud 2y ago
    The raw words written to the drive are actually re-encoded into slightly larger codewords with nice properties like not having too many zero or one bits in a row, and error detection/correction. Plus I think that the 0/1 bits are not encoded as "no magnetism"/"some magnetism", but instead as "north magnetism"/"south magnetism" since magnetic fields have a direction. And I don't think the magnetic fields on the platters have any appreciable effect on the head besides the electromagnetic effects at the sensor.
  - Cerium 2y ago
    Essentially the same problem as with fiber optics. The data can't be recovered unless there are frequent bit transitions. In that case the data is transformed to ensure illegal patterns cannot occur.
- perihelions 2y ago
  Also, you could imagine a magnetic bearing whose field strength rapidly diminishes as you move away from it, due to cancellation, like a Halbach array [0]. I wonder if there's a simple geometric configuration for a bearing that has this property, as well as the critical property of passively stable levitation, maybe based on diamagnetic or superconducting materials [1]. edit: Maybe something in this direction? [2] [0] https://en.wikipedia.org/wiki/Halbach_array https://en.wikipedia.org/wiki/Halbach_array [1] https://en.wikipedia.org/wiki/Magnetic_bearing https://en.wikipedia.org/wiki/Magnetic_bearing [2] https://ntrs.nasa.gov/api/citations/20070006849/downloads/20070006849.pdf https://ntrs.nasa.gov/api/citations/20070006849/downloads/20... ("Development and Testing of a Radial Halbach Magnetic Bearing")

Right you are, forgot about that completely! IIRC this is important to prevent a headcrash if the drive is jolted while in operation (of course within certain limits).

Brian_K_White 2y ago
It's required to work at all, not just as some kind of guard rail. It's like the oil in a normal bearing, the oil isn't just to prevent a crankshaft crash in case the car is jolted while in operation, the oil is a part of the way the thing functions at all in the first place.

ahartmetz 2y ago

Maybe magnetism could work. HDD platters take very strong fields to change magnetization these days, so erasing the data with the positioning magnet(s) seems avoidable.

amelius 2y ago
So write too many zeros to your drive, the disk becomes demagnetized, and the head will crash?
- Faaak 2y ago
  With another encoding like Manchester, it wouldn't;-)
- frud 2y ago
  The raw words written to the drive are actually re-encoded into slightly larger codewords with nice properties like not having too many zero or one bits in a row, and error detection/correction. Plus I think that the 0/1 bits are not encoded as "no magnetism"/"some magnetism", but instead as "north magnetism"/"south magnetism" since magnetic fields have a direction. And I don't think the magnetic fields on the platters have any appreciable effect on the head besides the electromagnetic effects at the sensor.
- Cerium 2y ago
  Essentially the same problem as with fiber optics. The data can't be recovered unless there are frequent bit transitions. In that case the data is transformed to ensure illegal patterns cannot occur.
perihelions 2y ago
Also, you could imagine a magnetic bearing whose field strength rapidly diminishes as you move away from it, due to cancellation, like a Halbach array [0]. I wonder if there's a simple geometric configuration for a bearing that has this property, as well as the critical property of passively stable levitation, maybe based on diamagnetic or superconducting materials [1]. edit: Maybe something in this direction? [2] [0] https://en.wikipedia.org/wiki/Halbach_array https://en.wikipedia.org/wiki/Halbach_array [1] https://en.wikipedia.org/wiki/Magnetic_bearing https://en.wikipedia.org/wiki/Magnetic_bearing [2] https://ntrs.nasa.gov/api/citations/20070006849/downloads/20070006849.pdf https://ntrs.nasa.gov/api/citations/20070006849/downloads/20... ("Development and Testing of a Radial Halbach Magnetic Bearing")