The Symptoms of Hard Drive Electrical Damage

by PlatterSwapper

One of the more common causes of failure in hard drives is the result of a spike on the power supply line. Sometimes this is the outcome when someone plugs-in the wrong cable (or the right cable but the wrong way round), sometimes it is due to a failing power supply within the computer or may simply be caused by an electrical storm in the area.

What Happens When a Hard Drive is Exposed to Excess Power?

The result can be damage to the printed circuit board (PCB) mounted on the hard drive chassis. Most hard disk PCBs are fitted with components which will sacrifice themselves in an attempt to protect the rest of the drive from damage in the event of sudden excessive current. Typically this will involve either fuses and/or protection diodes (transient voltage diodes, usually simply referred to as TVS diodes). The former will act like a switch and open (thereby preventing the damaging excess current from reaching the rest of the hard drive), the latter will form a short circuit to the chassis ground (thereby taking all of the supply current away from the hard drive to your computer’s ground, protecting the drive itself from damage). Both of these devices remain in the fault condition even after the problematic power has been removed. In other words, even after you switch your computer off, the fuse remains in open circuit condition and the TVS remains in short circuit state.

A TVS (on the left) and the fuse are identified with arrows

 

Both of these methods of protection take a finite time to act after the excess power has been applied to the drive and even within this brief interim it is very common for other components within the hard drive to suffer damage. The most common types of damage seen in these circumstances are additional component failures on the PCB (typically the components which regulate and modify the supply voltages for use by the hard drive can suffer) and damage to the read / write head pre-amplifier circuit. This last is a particular concern. It is vulnerable to electrical damage and as it is mounted on the side of the read/write head arm  requires the drive to be opened and donor parts in order to regain access to the drive data (for a quick guide to hard drive anatomy have a look here).

Where the TVS has been forced into short circuit it will typically be the case that the computer using that hard disk will refuse to switch-on (or alternatively will switch-on briefly but then quickly shut-down again).  The hard drive now presents a direct short circuit to the computer’s power supply and in order to protect itself it the computer will switch-off.

Where the fuse has gone into open circuit the computer will switch-on but the hard drive cannot spin-up of course as it is receiving no power beyond the fuse.

If you have access to a voltmeter with a resistance (or continuity) measurement facility then you can determine whether or not a fuse and/or a TVS have been damaged by direct measurement across the suspect component(s).

This type of damage can typically be worked around in order to regain access to a damaged hard drive (at least for long enough to recover a user’s critical data) however caution and thoroughness are required when diagnosing the extent of damage done to a drive by the excess power before proceeding as making the damage much worse in this situation is easily done.

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