One of the misconceptions about the low-level data storage is that the 0’s and 1’s are physically stored on the drive platter. The data is however encoded before being written to the platter as a wave form.
Prior to writing the data, it is randomised, which eliminates repeated patterns, which could cause problems whereby the Error Correction Code (ECC) becomes confused. As the drive firmware handles this automatically, in most cases it has no bearing on the data recovery procedure.
Physical Data Layout and Servo Data
Current hard drive technology lays the data down in a set of concentric tracks, around which servo tracking information is also stored, which the read/write head uses to ensure the alignment it correctly maintained. Each sector of store data also has an overhead associated with it, including Sync data, Address Marker, CRC and ECC, all of which are essential for correctly storing and accessing the data.
In the future, this tracking information may however be stored on a separate layer, allowing more data to be stored while improving the efficiency of maintaining the read/write head alignment. Future improvements may come from not only storing the track servo data as a separate layer, but also creating additional data layers on a single platter surface, which are accessed by focusing the read/write head to a different depth as required.
Drive Error Codes and Diagnostics
An extremely important part of the operation of a hard disk drive is how it handles any errors and how it interacts with the host computer system. Although called error codes, they also indicate if the drive is ready, busy or received a data request to or from the host computer.
In most cases, any real errors will be handled by the drive firmware, such as automatically mapping an unreadable sector to the spare area of the drive. If an error code is passed back from the drive to the host computer, it is essential that the operating system handles this in a sensible manner, without causing a system crash, which could result in unsaved worked being lost.
Data Recovery and ECC
At one time, it was possible to read the data sectors without invoking error correction, but even for data recovery purposes this proved to be of little use and is only available for drives less than 137GB in volume. By default, the drive will make several attempts to read each sector using different methods before it will timeout and return an error.
There are several techniques which our data recovery specialists can employ to read a previous unreadable bad sector, although each attempt may risk causing further damage to the drive if the problem caused a physical issue. Therefore, our data recovery engineers will maximise the yield of good data sectors before making attempts to read sectors which failed to be read correctly during the imaging process. In a number of cases our data recovery engineers have been able to recover all the sectors from a failing hard drive, despite a sector being unreadable during the sequential imaging process.