A small portion of a hard drive that, due to defect, can't hold data. SATA RAID units often contain technology which diagnoses and corrects bad blocks before they impact RAID array performance.

A bad block is the result of a media flaw or damaged format markings which causes the data held on a block (usually the size of a sector) to be unreliable.

(computer science) a block (usually one sector) that cannot reliably hold data