If the wall power cuts out right now, or somebody pulls the computer power plug out from the wall, there is no time for the mechanical hard drives to react to this event.

Won't this cause the HDDs' motors to slow down and both the arm and discs to get damaged because of the "vobbling" since it has not had time to get a "please unmount the needle" signal sent to it, as normally happens if you shut down the OS properly?

Also, if somebody bumps their running laptop with a (2.5") mechanical HDD in it onto a table or something, why doesn't that cause the arm and discs to touch?

Did they invent some kind of ingenious failsafe mechanism which mechanically moves away the arm from the discs instantly and with no power required if it detects an "abnormality"?

1 Answer 1


In most harddisk designs the mechanical arm is moved by an electro-magnet which is a series of wire coils wrapped around the arm.
The magnetic field from this electro-magnet pushes against the field caused by 1 or more permanent magnets around the arm.
By modulating the current through the coils the electro-magnetic field is changed to achieve the correct positioning of the arm.

The system is setup so that if the power goes out the permanent magnets pull the arm away from the spinning platters into a safe position.

Additionally there is an air-cushion between the heads and the fast spinning platters. Even if the power is cut the inertia in the spinning platters keeps them spinning for a few seconds which maintains that air-cushion long enough for the arm with the heads to reach the safety zone without the heads scrapping over the platters.
That same air-cushion provides some protection when a harddrive is bumped while being in operation.

Last but not least. Most modern harddrives have a buidin accellerometer to detect shocks. If that happens the drive-firmware will also pull the arm into a safe position as quick as possible.

  • 4
    Just to reinforce your comment on the drop scenario: a drop is typically two events as a hard drive would know about it. There is the sudden "loss" of gravity which is unlikely to damage a drive and is most likely when the drive will park the heads to protect the drive, and then there is the sudden impact which is what causes the damage. The heads should be able to park relatively quickly so a fall of a few feet may well be long enough for the heads to reach the safe zone.
    – Mokubai
    Sep 30 at 13:18
  • @Mokubai Very true. The old saying "It's not the fall that'll kill you. It's the sudden stop at the end." applies here too. But the fall can be detected and the falling time used to intervene. Doesn't do much for sideways or very short vertical bumps, but there the air-cushion will provide some protection.
    – Tonny
    Sep 30 at 13:22
  • 2
    Also, in electronics in general, if it is crucial for a device to have power long enough to prevent damage or a loss of some kind, one adds a capacitor to store residual power to give power when incoming power ceises. If we talk about an SSD, it would have a capacitor to finish writes if it needs to.
    – LPChip
    Sep 30 at 19:31
  • 3
    It is worth observing that in times past, drives did not have any form of auto-park. In particular, back in the mid to late 80's I worked on a system that had such a drive (20 MB IIRC), and we had to be sure to park the heads before powering off. And yes, if it had suffered a power loss, we'd have been in trouble. It probably won't come as a surprise that it was connected to a UPS.
    – dgnuff
    Sep 30 at 22:16

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