A reader asks: “My 500 GB hard drive has one bad sector; what does that really mean?”

A bad sector is a small area on the hard drive’s surface that’s unreliable or unreadable; and that the drive’s own software, or the PC’s operating system, has marked as off limits so no new data will be written there.

By itself, it’s nothing to worry about. For example, one sector on a typical NTFS-formatted drive holds just 4KB — that’s KB, not MB!

Most drives are built to cope with some bad sectors; they ship with spare sectors, just for this purpose: When a damaged sector is discovered, it can be marked as bad, and any data that was in that sector can be recovered and moved to a spare sector, which will then be re-mapped to take the place of the damaged sector. It’s usually all automatic and invisible, unless you go looking for it.

Finding one bad sector is insignificant; but finding many bad sectors, or a steadily increasing number of bad sectors, is a definite sign of trouble.

How can you tell what’s trivial and what’s not? Check the drive’s internal SMART data (more info). A free SMART app (examples) can show you an incredible amount of statistical detail about the health and status of your hard drive — sometimes even giving you an estimated future day, date, and time when the drive is projected to exceed its safe operating limits.

So: Don’t worry about a bad sector here or there. Absent any other indicator of trouble, it’s not a big deal.

Related info/more detail: Should you trust a hard drive after a major error?

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