Bad sectors and HDD failure
You might have noticed your computer performing slowly, becoming unresponsive, or even ‘hanging’ and restarting itself. This can be due to an excessive amount of bad software that loads at start up (see information on bloatware). However, most people don’t realise, until it is too late, that it is their hard disk drive that is failing. One of the most telling symptoms can be excessively long file transfer times – when you copy the data the progress bar tells you it will take several hours or days to copy the files. This is a sure sign that you have “bad sectors” on your hard disk drive.
What are sectors?
A sector is an individual area which can be addressed. Traditionally a sector is made up of 512 bytes. More modern hard disk drives using advanced format technology use a 4,096 byte sector. This has allowed efficiencies in storage capacity and improvements in error correction algorithms.
Assuming a 512 byte sector, a 500GB hard disk drive has a total of 976,773,168 sectors that can be addressed. This is also referred to as the total logical block addressing (LBA), which is a common scheme used for specifying the location of blocks on a hard disk drive or memory stick/card.
What are the causes of bad sectors?
General causes of bad sector formation are physical or magnetic corruption. Physical corruption is easy to understand – it occurs when there is physical damage done to the media surface. Magnetic corruption occurs when a hard drive miswrites data to a wrong location. While the latter may seem to be less damaging, it is actually as dangerous as physical damage, since miswritten data may damage not only adjacent sectors but also servo sectors.
As the hard disk drive ages its ability to read and write declines. Moreover, the chemical substrate on the platter surface that holds the electrical charge decreases gradually. If the electrical charges held in each sector deteriorate beyond the ability of the read/write heads correctly to decode the data then that sector then it can no longer be read reliably and is considered a “bad sector”.
We typically see unreadable sectors for the following reasons:
- Read/write head degradation or misalignment. In these cases, we often see only one platter affected from the weak head, whilst the others read perfectly. We can use disk imaging equipment like the DeepSpar Disk Imager or AceLabs PC3000 Express to copy data by head, producing very good results.
- Unreadable sectors after a power surge which has also damaged the printed circuit board.
- Degradation of the media (platter surface).
All hard disk drives have firmware. This is the software that controls the hard disk drive. There is a function that monitors the number of defects (bad sectors) that a hard disk drive has. When you buy a hard disk drive new it already has bad sectors on it! However, these have been added to the permanent defect list (P-List) in the firmware. The HDD knows to avoid these areas on the platter. The firmware also has a module called the “Growth defect list,” (G-List). This is a list where new bad sectors are added as the drive ages. Again the HDD will remap these sectors and no longer use the bad areas of the disk. There is a reserve pool of sectors to allow this. When there are a large number of bad sectors it can cause the firmware to fail as result of the G-List becoming full.
What to avoid doing
Do not run Microsoft CHKDSK or ScanDisk. The results can be disastrous. This software is designed to be used on hard disk drives that are working OK, but have a corrupt filesystem. The end result of using this software can be that your critical data can be converted into .CHK files.
If you are extracting your data and the progress bar in Windows or Mac OSX is telling you that it is likely to take several hours or days to copy the data then please turn off your computer. When your operating system cannot read data from a bad sector it makes multiple read attempts. Performing excessive read attempts can cause the read/write heads to degrade or the media surface. In short, it is likely to make matters worse. Please contact us and we can examine the drive before it degrades further to allow the best chance for a successful recovery.