(this is going a bit off topic but I'm genuinely curious)
free_electron, I've often wondered why hard drive manufacturers wouldn't design a hard drive that would have a head stack on each corner of the unit, so four in total. Just read either four tracks at the same time or a single track using all four heads and assemble the data into the cache of the drive in a
mojo_chan : hard drives fail in 3-5 years even without keeping them on 24/7. I run my system 24/7 on and have four drives in the system, oldest is about 4 years old and works fine. I replaced one recently due to starting to develop bad sectors though.
I agree with your on just the comment about lightning strikes or fires.. that's still a risk.
That is an old idea and has actually been tried , not with four but with two headstacks at opposite corners. You cant do four courners as the heads would bang i to each other the arc on which they move is made very large as to keep the head perpendicular to the track at all times. Now that we have dual actuators this could actually be attempted.
But there are other problems. You still have to wait for data to fly by underneath the head. So you could not use both heads simultaneously. So throughputwise you gain nothing. You could maybe seek a bit faster. While stack 1 is reading stack could be seeking the next block. But the amount of mechanics and the associated cost and failure modes make this unusable. Electronics cost nothing. 90% of the cost of a drive is mechanics. Of that cost 30% is the media (platters) 60% the headstack and its actuator and the rest the motor and case.
In short : you gain nothing and only drive up the cost. Other factor is the required physical room for a second headstack. This means smaller platters thus less capacity. Its a loose loose situation.
Dont confuse dual headstack drives with dual actuator drives. The dual actuator is a second 'motor' o. The headstack that can flex the head.
Here is the problem : the heads are at the end of an arm. This arm moves around a fixed point. So the heads essentially move in an arc. If you position the heads in the center of the usable space between hub and rim of the platter , the heads are perfectly positioned on a track. Move the head outwards or inwards from this center and the head now sits under an angle on the track. This the track will be wider !
So what we do is mount a piezo element at the tip of the arm where the head is. By flexing the piezo we can twist the head position so it becomes perpendicular again.
Imagine it like this.: stretch out your arm , hand flat , fingers pointed out in fro t of you. Draw parallel lines with your arm. Your fingers are perfectly aligned with the line in front of you. Move your arm left or right without bending wrist or elbow. Your fingers now no longer sit parallel with the lines, they sit under an angle. Flex your wrist to adjust your hand. That is what the second actuator does. It gives the head a 'wrist'.
This is done to make the tracks uniform in width thus upping the total amount fo data we can cram on a platter.