Bad Disk Sectors Explained


Bad drive sectors (much like bad weather) are practically unavoidable - mostly because they are caused by so many different things. As saddening as those news may be, things do get better once we take a closer look at the matter. Like with the weather example, the key here is prevention. Before you rush to the store and get your drive an umbrella, let me clarify: I’m talking about Backup!

class-100-clean-room-hard-drive

Let’s take a look at what bad sectors are.

What is commonly known as a bad sector is really just a tiny area of a drive that for some reason cannot be read or written to. There are two subdivisions: “hard” bad sectors are caused by physical damage, while “soft” bad sectors (as you might be guessing) are caused by software errors.

Hard Bad Sectors are generally created when damage has in some way been dealt to the disk itself (the image above should illustrate how easy that is in magnetic drives). Common scenarios include bumping the disk while it is in the middle of an operation (making the read/write head scratch against the surface of the drive), allowing dust to penetrate the interior of the disk, natural wear and tear and manufacturing errors. These are generally quite hard (or, in some cases, impossible) to fix. Obviously, only some of these scenarios apply to Solid - State Drives as they have no moving parts.

Soft Bad Sectors are often an easier beast to tame, as the “error” resides in the logic (software) only. They are usually caused when one shuts off the power to the disk in the middle of a write operation, or when for some other reason the data on the sector does not match the “error correction code”. Soft bad sectors are sometimes caused by malware, too.

Solid - State Drives are a special case: firstly, the manufacturing techniques have not yet been perfected - resulting in brand new disks often being unusable. Secondly, because of the nature of the technology used in SSDs, the memory units degrade with time - and thus bad sectors are practically “part of the plan”. SSDs now come with “extra” storage capacity, which is used to maintain the disk’s capacity even after several sectors have been deemed unusable. When the extra space runs out, the storage capacity of the drive starts to degrade.

Generally, modern computer systems are smart enough to determine that a specific sector is faulty and, if possible, move the data elsewhere, marking the faulty sector as a  sort of “no-fly zone”, allowing the user to happily live on as if nothing had happened.  If needed, one can sometimes repair those using the tools like “Disk Utility” in Linux - based systems or “chkdsk” in Windows. It is important to note here that contrary to popular belief, a single bad sector does not suggest that a drive is about to fail any more than a hot day’s weather indicates that it will rain fireballs. Do keep an open mind and a consistent backup though, as anything is possible.