NT Disk Viewer is a free tool for working on hard drives, as it lets you view the information on each sector of your hard drive in hexadecimal or ASCII.
In a very visual way you can see each one of the sectors and check its status. NT Disk Viewer lets you copy and export the sector information to files. It supports working on both FAT and NTFS file systems.
A highly useful tool for technical specialists who sometimes need to physically access each sector.