Re: Hey Mamr
Load/ Unload means head loading onto the platters and retracting them off into the parking garage.
Modern drives no longer land the heads on the platters on powerdown or idle. The heads are retracted and slide into a 'garage'. This is technically a misnomer. What happens is the arms pull back and slide onto a ramp. the lifts the heads up and away from the platter surfaces. The arm then lands in a notch that holds it. The heads are dangling free.
A headstack 'parked' this way is very resilient towards damage and shock. There is no risk of the heads banging into anything and chipping the ferromagnetic surfaces.
Whenever a drive is not used for active data transfer a timeout system will command the headstack to retract. The motor keeps spinning so the platters remain up to speed although they can 'float' a few thousand RPM. They will stay above a certain minimum so the heads can be 'loaded' at will.
A minimum RPM of the platters is required to create enough surface pressure to keep the heads floating and not slam into the surface.
Another common misbelief is that people think the heads 'crash' into the platters and cause damage. A head crash damages the HEAD. The surface of a harddisk is so hard (made from diamond-like carbon deposit. it is actually a carbon layer grown in a diamond lattice) you cannot scratch this. the magnetic layer is buried under that surface. the top of the carbon layer is lubricated with a coating to increase surface tension. This repels the heads away from the surface.
The spinning platters create a suction effect by pulling air between the platter surface and the head. It is basically an airplane wing upside down. The heads contain a heater element. By altering the temperature of the head they can control the flying height. As the head is writing it also heats up and the pole tips extend. By altering the heater temperature they can compensate for this. Control algorithms seek for maximum signal strength and not only drive the head towards the center of a track but also seek in vertical space for optimum flying height.
I worked for years in the HDD world and it still amazes me how they can cram all that data on such small a surface with this reliability. Some of the stuff they do is like Evel Knievel doing motorcycle stunts...
Prediction algorithms figure out what will pass at what moment in time underneath the head and steer the headstack to the right spot , in time for the data sector to fly by. the head accelerates, maintains speed , then decelerates just in time to pick up the 'pilot' that precedes the data block. The pilot is phase locked to fine control drive rotational speed. depending on the track we are on we know exactly how many microseconds of data will pass by before we will hit another pilot block. The drive times all that stuff out and then turns on the writer for that precise amount of time to spit the data onto the disk.
While writing a block the drive is 'blind'. It all depends on accurate timing so not to write beyond track end, and accurate positioning , so not to write into adjacent tracks. It's like doing a run around an athletic course, you have to run 100m in 10 seconds , staying in your assigned track . stopping just short of the finish line ( stepping on the finish line corrupts the pilot, going beyond throws data in the next block corrupting that) , all while blindfolded.
Biting the hand that feeds IT
