I understand that if the hard disk's firmware had problem accessing/writing/reading a sector (physical bad which is unrepairable, or logical bad which is repairable), the sector will get reallocated.
Now, lets say that you would like to scan that sector, after it had been reallocated, in case it's repairable.. how could you do that when the software can not reach that sector due to its reallocation?
If it's hard drive firmware related where the firmware ID's a bad sector and reallocated a good sector for that space, I don't think that's easily doable.
If it's OS software related, you can run Chkdsk /b with the /b switch, which implys /r, and will unmark the bad sectors, to recheck to see if they can be written to and read from accuratly. It won't just check one sector, checks them all and attempts to recover potentially good space.
Those sorts of functions are only accessible via Vendor Specific Commands or via a diagnostic console. Many drives have a serial port for this purpose, while others can also use the SATA or IDE interface.
For example, WD has an internal tool called Trex. Professional data recovery tools include PC3000, Salvation Data's HD Doctors, HRT, and various other cheaper tools including DFL, SRT, WDR-UDMA, STR3000-UDMA.