20 March 2012

One of the most common symptoms that data recovery companies see in their labs is a hard drive that is clicking and spinning down. In the vast majority of cases, the problem is that one or more of the heads is bad, and the head assembly must be replaced from a matching donor for full recovery.

However, with the right tools and technology, a technician can often initialize the hard drive and, at the very least, read data off the good heads without even opening up the hard drive cover. If a hard drive has eight heads, which is a common situation nowadays, it’s possible to get data from seven out of the eight heads (87.5% of all sectors). Reading data off good heads before ordering the donor for heads replacement is crucial, and sometimes even enough for complete recovery.

The idea behind this strategy is simple.

Say we have eight physical heads in the drive, numbered 0 to 7, so the resulting map of used heads looks like 0,1,2,3,4,5,6,7. If a single head is bad (which is very common in the real world), such as head #7, the drive just clicks a few times and spin down. Obviously the drive won't ID and won't be able to read any data.

Now, here's the trick. Using a specialized utility, we can modify the hard drive internal RAM memory and “shift” that bad head #7 in the head map to one that is in good shape. So if we change the head map to 0,1,2,3,4,5,6,6 and instruct the drive to reinitialize, it spins up and starts just fine with the new head map in RAM.

The next step is to put the drive on DeepSpar Disk Imager and image all sectors that belong to heads that are good (0 to 6), leaving sectors that belong to head 7 for the next step.

As mentioned, it does make sense to analyze the partial image after acquiring data from the good heads. Depending on what kind of data the client is looking for (and with a bit of luck), there is a good chance that all the needed files are already recovered and we don't need to swap heads in the cleanroom to finish the imaging process.

Even if the needed files are not there and the full image is required, acquiring a partial image before taking the drive to the cleanroom can still be very useful. With growing magnetic density and precise hard drive mechanics, perfect donor drives are getting harder and harder to track down and head swaps are harder to achieve.

After swapping heads from a matching donor, you may find out that your bad drive is still clicking with the same symptoms. In reality, some other physical head may be causing the clicking. So initializing the drive by modifying a head map in RAM may allow you to get the drive to read using the previously missed head so you can complete the image.

Without this technology, your only other option is to keep searching for that perfect donor drive with a head assembly that works for all heads . . .