Why Do Disk Errors Occur?
In addition to Disk Utility, numerous third-party utilities check for, and attempt to repair, a wide range of disk errors. I've run these utilities hundreds of times over the years, and a shockingly large proportion of the time they've found and fixed errorseven though I wasn't aware of any problem. What causes these problems in the first place?
Although I can't give you a complete answer to this question, I can offer a few reasons some disk errors occur:
Buggy software: Virtually all software contains some bugs, and bugs can cause bad data to be written to your disk or can corrupt existing data.
Hardware failures: If your hard drive, logic board, or various other components have manufacturing defectsor develop malfunctions later onthese can result in disk errors.
User error: If you unplug an external hard drive without unmounting the volume(s) first (File > Eject), you may interrupt the flow of data to the drive, or interrupt a housekeeping procedure. Many other user errors can also result in disk problems.
Power failure: If the power goes out (or your battery dies) at an inopportune moment, a file may not be written properly.
In other words, stuff happens. You can guard against some problems simply by being careful, but stuff will still happen.