I was recently discussing hard drive errors with someone and they came up with the idea of zeroing out a hard drive to fix the bad sectors. Zeroing out the disk may appear to fix the issues for a while, but I can guarantee that the issues are still there and will only get worse.
Once you see a bad sector on a hard disk, just one, it’s time to backup your data and throw the disk away. If you don’t want to junk the disk, at least use it only for data you don’t care about losing.
You see, the way modern hard disks work is that they all have bad sectors. In the good old days, hard drives used to come with a printed list of known bad sectors and when you did a low level format on them (a real low-level format, not just zeroing out the data) you would then manually enter the list of known bad sectors, and then update that list with any new ones – in other words, you were in complete control over mapping out bad sectors.
With today’s hard disks, the hard drive controller maps out bad sectors itself, without even telling you. It has a factory list of bad sectors, that you can’t see. As it encounters bad sectors in normal use, it silently attempts to recover the data and move it to a chunk of spare sectors that all drives have for this purpose. You generally aren’t aware that anything has happened. There will be a SMART status code logged, but OS X doesn’t seem to regard this as an issue worth reporting. You can use something like smartmon tools to check the raw SMART status and see this for yourself.
Once a hard drive has mapped out enough bad sectors that it’s starting to report to you there are bad sectors, this is the beginning of the end. I have not once had a hard drive last for a long period of time after reporting a bad sector, it always gets worse and worse.
The best thing you can do is to power down the drive immediately and let it cool down to room temperature. Get something to back up your data to and begin the backup process straight away.
Zeroing out the disk may let the drive remap a few more sectors and give you the illusion of having everything fixed, but it’s a very different picture behind the scenes.