Micro-Jog, A Data Recovery Introduction
‘MR Head Deferential Micro-Jog’ to use it’s full name, since being introduced by manufacturers to both increase read/write stability, improve platter density and I/O speed has been a vital information set for data recovery engineers. This adaptive information is vital to allow technician’s to determine if a donor set of heads are compatible to replace a set of failed heads during a read/write head transplant.
The basic explanation of micro-jogs is the distance between the read head and the write head. This micro measurement is used to slightly move the centre of the read/write head over the centre of the track. Micro-jog information is calculated at the point of manufacture and stored along side other vital positioning information to provide fast and reliable read and writes. A portion of this positioning information can be extracted and used by data recovery experts to locate a suitable head stack assemble (‘HSA’) donor for a failed hard drive.
Case Study – Recovering A Hard Drive Using Micro Jog
In this case study a customer’s hard drive had been diagnosed with a failed HSA. It was therefore necessary to replace the faulty HSA with a working one from a suitable donor drive, ensuring that the donor drive had suitable micro-jog values that made it compatible with the faulty drive.
Our technicians read the EEPROM from the failed hard drive, extracting the micro-jog values and storing them for reference. Each head within the drive has a unique micro-jog value (stored in Module 47 within the EEPROM). Once we know the faulty drive’s micro-jog values we were able to compare them against the values from the donor HSA’s and identify which drive has the closet micro-jog values and is therefore the best match, and thus provides the best chance of a successful data recovery.
Unsuitable Donor Drive
Our case study shows the extracted micro-jog values for the failed hard drive and the details for a similar drive we located in our stock of spare parts:
To ensure the best possible chance of recovery it is advisable that the difference between each head’s micro-jog value should ideally be no more than 100 and with an absolute max of 300. Our own internal testing shows that when the 200 mark is exceeded the head will exhibit read/write errors and a significant reduction in speed. As we can see from the example above, 3 of the 6 heads on the donor have a difference value of >100 with one head (Head 0) displaying a value above the 300 threshold, as a result this donor did not have a suitable HSA for our failed drive.
Finding The Right Spare
Fortunately we were able to find a donor drive with much closer micro-jog values – see below:
As you can see, this donor was a much better match with only one head having a difference value of 100+, this drive was an ideal donor drive for our faulty drive.
Acceptable Micro Jog Differences
If the difference in micro-jog values between the faulty and the donor drive is greater than 200, specialist equipment can be used to manipulate the micro-jog values to create an ‘average’. This can help with cloning speed and reduce errors, but will not help if the value is above the 300 threshold, it is however advisable to find the best possible donor as manipulating micro-jog values to create an average is often a last resort.