Several years ago, my old XPh system began behaving erratically. I finally decided to run chkdsk on the C drive, which I did from the command line, with the Repair switch.
Chkdsk indeed found disk errors, and I watched in horror as it began disallocating sector after sector. A huge portion of the drive was being taken offline before my eyes, and there seemed no end in sight. I intuitively felt that the drive really wasn't in that bad a condition to warrant this kind of action, and considered that I either was losing valuable data or soon would be.
I took the panic response and aborted the scan, hoping for the best. And I was glad I did. A 180G drive had lost some 30G to the disallocation, but my system ran fine on the now-reduced-size drive. Years later, after I took that old drive out of service I put a partition manager on it, re-allocated that lost space, reformatted, and the drive is full-sized and perfect again.
Now here is my question. Why did ChkDsk act this way? I never trusted it after that. Only recently, when I transferred to my new system, did I begin to use ChkDsk again, and it's performed fine. But I haven't forgotten the time it almost wasted my system, so I'm concerned about safety.