When the rebuild is complete, you'll need to restart the computer. The contents can become corrupt, and that can cause mysterious things to happen. Instructions are included with either utility. Use the Directory Maintenance feature of TechTool Pro to diagnose and repair your hard disk directory. The directory can become slightly damaged by such events as a power failure, a crash, or the need to force quit an application or a forced restart of the computer.
Slight damage can, by itself, lead to serious damage and the loss of data.
Directory Maintenance is a new feature of TechTool Pro 3.0.1, and I have found that it has repaired damage that earlier versions of TechTool, and the current version of Norton Utilities, simply won't repair. While searching for and repairing any damage, it also defragments the directory files and freespace. If you perform Step 3 first, you'll probably find that the file structure is undamaged. But check it anyway. Optimize your hard disk Fragmentation of files and of the unused space on your hard disk is inevitable.