MHDD is the most popular freeware program for low-level HDD diagnostics.
MHDD supports these interfaces: IDE, Serial ATA, SCSI. Also there is a possibility to access an USB storage, there are drivers for emulation (USB->SCSI).
This software can make precise diagnostic of the mechanical part of a drive, view SMART attributes, perform Low-level format, bad sector repair, different tests and tens of other functions.
MHDD is the small and powerful freeware tool to work with hard drives at the lowest possible level. First version was released in 2000 by me, Dmitry Postrigan. It was able to make surface scan of an IDE drive in CHS mode. My main objective was to develop well-known and trusted HDD diagnostic tool.
Now MHDD is much more than just diagnostic. You can do anything with MHDD: diagnose your drive, access raw sectors, manage S.M.A.R.T. (SMART) and other drive parameters such as acoustic management, security, Host Protected Area. You can even send your own commands to a drive using very simple linear scripting engine.