The most important tool you MUST have if data is irreplaceable
Understanding the issues related to bad sectors on a hard drive.
What casues a bad sector?
Bad sectors can happen from physical damage on the platter's surface, a faulty or failing read write head or system area corruption, which can also cause a sector to fail the ECC error checking control checksum.
Remember if data is important please consider the experts!
What are Sectors and Bad Sectors?
A sector is the smallest unit of data on a hard drive. A drive's platter is divided into tracks, and then the tracks are divided into sectors. Each sector has a specific location called its Logical Block Address (LBA). A bad sector is a sector that has damage and can no longer be used to store data. Bad sectors can occur through normal use of a hard drive and a properly functioning drive has a built-in process to work around them, the data that runs this process is stored in the drive's system area. When a hard drive finds a bad sector it will reallocate it; it moves the bad sector from its current LBA and places it in a defect list (a section in the system area designed to record defects), it then copies the data from the bad sector to an empty and working sector.
Causes of Bad Sectors
Failing read write head
A read write head that is failing and unable to read a sector or an entire platter will label the sector(s) as bad, this is a 'false read' because the problem is not the sector, it is the read write head. This type of failure is very common.
Corruption of the sector location data
If the LBA is lost, the sector can no longer be located and is marked as 'bad'.
The defect list is full
A hard drive's defect list is allocated a small portion of the drive. If a drive has too many defects and the defect list reaches its capacity, the drive can no longer reallocate the bad sectors because it has nowhere to store them. Note: this can sometimes cause a drive to click or be inoperable.
Physical damage to a sector caused by dust or particle contamination
If a drive is opened outside of a clean room environment, the dust or other airborne particles in the air land on the platters. A hard drive has millions of sectors therefore an individual sector is extremely small and even a microscopic particle can physically damage it. This is why it is very important not to open a hard drive if your data is important, take your drive to a company with a certified clean room. Removing the lid for just a couple of seconds is enough to cause contamination.
Locations of Bad Sectors
The location of bad sectors on a hard drive is very important. If there are bad sectors in the master file table it can cause damage to the data structure. The master file table contains files names, the physical location of the drive's data and their related LBA's. The master file table with a NTFS File system or MFT is normally located around LBA 6,300,000; you can find exactly where your drive's file system is located using tools like Winhex.
If you image a hard drive that has thousands of bad sectors in the master file table, you would have raw data. There would be no access to file names or location, such programs as raw recovery or fast file finders are good to use in these situations.
Diagnosing Bad Sectors
If a drive appears to be functioning normally (meaning the drive is spinning and there is no audible 'clicking' or 'beeping' noises) yet it is not operating correctly, the fault is most likely bad sectors. However, in all instances it can be difficult to determine the exact cause of your drive's failure because many hard drive faults have similar symptoms. Diagnostics are needed to establish a prognosis.
There are several diagnostic strategies that can determine the exact fault for data recovery purposes.
Checking the platter's surface for scoring, scratching and contamination (should only be done in a clean room environment)
Sector reading with sector read analysis. That is evaluating the time it takes a drive to read a sector in milliseconds and then disabling the error read control to read a known bad sector and comparing the results. Multiple comparison reads of different sectors are needed to get the most accurate results.
Remember, if a hard drive has bad sectors or is otherwise failing, it will worsen with more use. If your data is important, call professionals.