Why Swapping PCBs is Like Tearing A Tenner in Half

by admin

One of the more widespread and understandable misconceptions in hard drive recovery is that swapping PCB (printed circuit board) on a hard drive is all that is required to fix a drive with an electrical problem.

It makes perfect sense on the face of it. You can see the blackened component where the smoke escaped and the drive isn’t spinning-up so you reckon the fault is on the PCB. There is a good chance that you are correct (there is a also a good chance that damage has been done beyond the PCB itself, but for the sake of this article let’s assume that the damage is indeed purely restricted to the PCB).

You buy in a replacement drive (you acquire exactly the same model, the PCBs are “identical”). You swap the PCBs, the drive spins up (hooray! someone up there likes me), but then you hear

click, click, click… (aaargh!)

…and the drive isn’t seen in BIOS. It hasn’t worked.

Why has this happened?

To understand why, you need to know a little about how a hard drive gets itself initialised and presents itself to the outside world (i.e. to your PC or Mac). The key is the hard drive’s firmware, what’s firmware? – thrilled that you asked:

Firmware is data stored internally within the hard drive in order to get itself started. It cannot be accessed without specialised hardware. Any professional data recovery service will have access to and proficiency in the use of such equipment. It will include (among many other pieces of information):

  1. Drive Model
  2. Serial Number
  3. The location of the storage areas on the data platters that will be utilised to store user data
  4. Over the drive’s lifetime as individual sectors become unreadable the drive will reallocate the space lost with new space from a limited bank of spare sectors, the firmware keeps a map of these re-allocations.

To complicate matters this firmware is stored in part on the PCB (usually either on a discrete EPROM chip or else embedded within the PCB processor chip) and in part on the data platters. At initial power-up of the hard drive the contents of the PCB firmware store are read, this portion of the firmware will then direct how the drive starts up, this includes where it will look on the data platters for the remaining part of the firmware.

In some instances the starting location of the data platter portion of the firmware varies from one individual hard drive to the next even for the same model of drive (only the manufacturer could tell you why this is) and this random starting location is stored in the firmware on the PCB.

EPROM Location on a Western Digital PCB

Red arrow shows the storage location of the PCB portion of the firmware for this particular drive.

The firmware contents and the manner in which they are distributed between the PCB and the data platters varies greatly from manufacturer to manufacturer (and even from model to model), but the following is universally applicable:

Your hard drive’s firmware is individual to your drive, it is split between the PCB and the data platters and it must be readable on power-up or there will be no access to your data.
From this you can see why a simple PCB swap will not work.

The PCB and the platter firmware are like a ten pound note – if you tear it in half and try to match it with a half from a different note it may look OK but try spending it.

When it comes to recovering from data loss there are no do-it-yourself options for this type of problem, you need an experienced hard drive recovery expert with the tools to transfer the unique PCB firmware to a donor drive PCB.

{ 1 comment… read it below or add one }

Dan March 22, 2013 at 11:49 am

Nonsense, I just did the same thing with a Samsung Hard Drive and I had zero issues. Why would I give the drive to someone who had all the access to my data? I had a 2 day window where I didn’t have a backup and lost the data.
Rule is always backup your data, much cheaper than finding a PCB or paying a third party to recover data…

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