Hard Disk Monitoring

Hi

I am curious, as I have a hard drive that is slowly failing by showing more and more realocated sectors, how those realocated sectors are "made" and detected. I mean from the hardware/SMART point of view, how does it work? How a sector becomes bad, does it become during reading/writing or just with time?

As for detection, is it that, once a sector is written it is then verified after the writing, and if the test does not pass it is marked as bad, and then realoacted? Or is it that when the sector is read and if it's bad then it is realocated?

Just was wondering

cheers
Lucas

During normal operation of the hard disk, the drive is constantly analysing the disk surface.

If it finds a problemmatic area (one or more sectors where the data is not possible to access in a pre-defined time), it tries to read the data and copy it (reallocate) to the spare area. This may or may not complete successfully (depending on many factors, eg. how seriously damaged the surface or heads, does the spare area have free room for reallocation, etc.) During that time, the hard disk is usually not responding, the system (or at least software/OS trying to access the affected drive) seems frozen.

After the procedure completes (if things go fine), the original location is then (internally) marked as bad and all further read/write operations pointing to the original location is then redirected to the spare area.

So when the operation is completed, the original (bad) area is not accessible by software any more. Even re-install or complete re-format (or "low level format") will not show problems because the original bad area is not used any more. That's why in this case software (for example scandisk but even the Disk -> Surface test in Hard Disk Sentinel) will not find and report problemmatic sectors.

However, the reallocation process should not be ignored. If the drive status is stable (no further sectors will turn to bad) and the number of reallocated sectors is relatively low, then the drive can be used because the problematic area is no longer accessed.

However, higher number of reallocated sectors mean more damages of the surface - and this usually create even more and more reallocated sectors which all risk data loss and cause system instability issues.

You may find more information at www.hdsentinel.com/smart (for example in "Bad sectors?" topic)

Usually, if the drive is working in perfect environment (proper cooling, proper connections, stable power supply and power line, no shock / vibration, etc.) the number of such reallocated sectors may not neccessarily increase over time.
If there're no mechnical problems with the drive, each sectors can be read and written unlimited times (these operations do not cause a sector to be bad).

This is why you may find 100% perfect drives even after years of 24/7 operation.

Awsome!

Thanks a lot for a detailed answer