Micro-Jogs and the use in Data Recovery
The “Micro-Jogs” of a hard disk drive refers to the distance between read and write head elements calculated by the manufacturer. The values are used to slightly move the centre of read/write heads over the centre of the track for fast and reliable reading and writing. Essentially, the values are used for ‘fine positioning’ of the heads. Micro-Jogs are primarily found in Western Digital Drives.
The Micro-Jog values are calculated during the manufacturing process. These values are stored within the contents of the ROM.
When working on a WD disk we extract the contents of the ROM and check the values of the Micro-Jogs. Once we know the values for the failed (patient) HDD, it is possible to compare these to the values in donor HDDs. Having a good match of the Micro-Jogs in the patient and donor HDDs allows for the best possible compatibility in the head-assembly.
Case Study – When Micro-Jogs were critical to recovery
A Western Digital 500GB WD5000AAKX was being worked on as a result of a failure of the head assembly. The patient’s read-write heads/sliders were severely misaligned. Such severe misalignment can lead to physical damage to the platter surface, which in turn can make data recovery impossible.
Tolerance in Micro-Jog parameters
Typically, any patient and donor HDD should have micro-jog values within 200 of each other, across each of the read-write heads. Values of up to 300 are acceptable, but can potentially be problematic. Differences between head micro-jogs with values higher than 300 usually do cause problems, either with complete lack of functionality, or very poor performance.
Incompatible donor HDDs
Typically when locating a suitable donor Western Digital donor HDD we look for a match on the model number. An exact match on the model number in itself is not critical, but it tells us which engineering revision the HDD is from. In this case the Tahoe LT series. Next, an inspection of the “DCM” configuration, listed on the manufacturer label. In older generations of the HDDs, this used to critical to match parts of the code between the patient and donor HDD. However, we have found that for disks manufactured from 2010 onwards this has become less important. Nevertheless, it always helps to have it match than not.
Given the DCM code on our selection of donors, we decided to use a donor which matched the DCM, family series and firmware revision with that of the patient HDD. However, the Micro-Jogs had a difference of between 400-500. Use of this donor head-assembly allowed for partial calibration of the HDD, but it was not possible to read the user area of the platter. It was possible to read some firmware modules, but physical read errors were occurring. Testing with another HDD with similar micro-jogs to the first donor replicated the symptoms.
The correct donor HDD
Given the above, we looked for donor HDDs with a better match on the Micro-Jogs, even if other parameters (like the DCM) were not a good match. Use of donor with micro-jogs between 0 and 200 for the heads improved matters significantly. With this donor head assembly, it was possible to initialise the HDD fully and read the service area. Moreover, the HDD read the user data area well, with just a small number of bad sectors.
An alternative to a good donor – Micro-Jog averaging
In the case study above, it was possible to locate an HDD with close enough Micro-Jogs. However, in some jobs, it is simply not possible to locate a suitable donor. If this is the case then we can re-program the micro-jog settings in the patient, to match more closely the micro-jogs of the donor. This is known as “Micro-Jog Averaging”. This is performed within PC3000 and can allow for. The results of this can be unpredictable, which is why it is always better to find a donor which has a close match on the patient HDD’s micro-jogs.
Advanced Data Recovery Services
Cheadle DATA Recovery Ltd provides advanced data recovery services and frequently recovers data from hard disk drives which other data recovery companies do not. Each month CDR receives multiple disks which have been opened at other data recovery companies and have been declared as ‘unrecoverable’ due to either lack of appropriate work or donor parts. If you have important data you require recovering please contact CDR.