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

Be sure to back up all data before proceeding!

1. Replace the drive.

2. 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. If writing zeros to the disk does not resolve the bad blocks, then the drive will need to be replaced. Note also that this is usually only effective when there are a very small number of bad blocks, and that this solution is also typically temporary.