Assuming that you know the make of HDD in question, hit the manufacturers site. Grab up one of the low level format utilities they offer. Should clean the disk out good as new. When you offer it up for sale, no worries, the new owner can format/install to meet their own specifications as it will be treated as a spanking new disk. This is just me though. If I buy a disk, I want it clean. If you install an OS, and then sell the disk, you put that new owner in a tough spot to get support on the OS should they need it.