If the Surface Scan is finding bad blocks, this means that the drive is physically damaged. Bad blocks are sectors on your hard drive that can no longer be written to. Most often this is due to physical flaws on the surface of the disk platters, and as such cannot be repaired. Modern hard drives automatically manage these sectors, saving them to a cache so that they are not written to. When the Surface Scan begins to report bad blocks, it means that this cache has run out of space, and the disk may begin to write files to these portions of the disk, which will render the files unusable.

If Surface Scan finds bad blocks, there are basically two options.  

Be sure to back up all data before proceeding!

1. Reformat the drive and zero all data.

Modern hard drives have a spare sector pool where bad blocks are recorded so that the hard drive does not write to them.  In some cases this pool becomes full and TechTool Pro will begin to find bad blocks during a Surface Scan.  If the disk is erased by writing zeros to every sector, this causes the pool to be re-written, and in some cases this will free up space and restore the drive to normal activity. However, this is often just a stop gap measure, and in many cases the disk will begin developing more bad blocks soon.

2. Replace the drive.

If writing zeros to the disk does not resolve the bad blocks, the best advice, is to update your backup and replace the drive.