Data Recovery Weekly

Theoretically, data recovery tools are read-only that means that their usage cannot cause any damage. However, in practice, when recovering data you may observe the effect as if the hard disks are being destroyed mechanically. For example, we took four disks from the NAS, connected them to a PC and launched the RAID recovery tool. Immediately S.M.A.R.T. monitoring software (Cropel) raised alarm because too many seek errors arose. Indeed, these seek errors were caused by the disk vibration that was provoked by the RAID recovery command to move disk heads on all the disks simultaneously. To tell the truth, a NAS device does the same when reading data from an array. However, a regular NAS device is equipped with more vibration-resistant drive mounts and fastenings. So, when transferring disks from the device managing RAID array to a regular PC, you may get alerts from S.M.A.R.T. monitoring software telling that the values of the Seek Error Rate attribute have changed significant...

Looks like our S.M.A.R.T. tool at www.cropel.com might be ready for slow-ish initial release.

One should prefer to look at and to work with raw data, if at all possible. We got an example recently when an owner complained on a hard drive because SpeedFan reported Fitness around 25% . This does not sound good. However, closer examination revealed that SpeedFan conclusion was based on an attribute with a value of 100 (raw value 0). There appeared to be no trouble at all from the raw SMART output. Upon further investigation I found that there are two versions of the firmware on the same model hard drive, one reporting a perfectly good condition as value 253 (raw 0), and the other reporting the same perfectly good condition as value 100 (raw 0). SpeedFan sees both values as originating from a single model, and decides 100 to be a failure indication. The more complex interpretation becomes, the more suspectible it is to all sorts of quirks and glitches. Unfrotunately, the more complex interpretation requires more human effort to work from raw data, but that's another ma...

Seagate drives are known to report exacerbated S.M.A.R.T. data for Raw Read Error Rate. This is well-known, normal, and should just be ignored.

It was previously thought that there was a clear correlation between disk temperatures and failure rates; however, the studies undertaken by Google Inc. on the large disk population have revealed that the correlation is not as strong as it was assumed earlier. In the studies, S.M.A.R.T. data which were collected every few minutes during 9-month window of observation have been analyzed. Only average temperatures were taken into account. It was found that failures don't increase when the temperature increases. Moreover, the higher probability of failure rates was observed for the lower temperature ranges. The positive correlation has been detected only for the disks with temperatures greater than 50 0 C. However, 3 and 4 year old drives stand out. For such drives, the correlation between average temperatures and failure rates turned out to be more pronounced, probably due to then current HDD technology. Thus, the studies show that the disk temperature affects the failure rate directl...

Since it is known in advance that it is impossible to create a perfect magnetic surface, a number of spare sectors are reserved on the drive. When a surface defect appears, the sector with the defect is replaced with a good one from the pool of a reserved sectors. Obviously, there is no surface repair involved. Instead the special record is made in the address table, like "if the write/read request arrives for the sector 123, use the sector 456 instead". This results in a certain loss of performance because it is now required to move the head to the reserved sectors zone and back again instead of just reading a contiguous chunk of data. On top of that, the data which was stored in the bad sector is lost. Nevertheless, theoretically you can use the drive further as if there are no bad sectors at all. This process is called “ reallocation ”. The S.M.A.R.T. attribute named “Reallocated Sectors Count” shows a number of the reallocated (replaced) sectors. If the drive idles long e...