Most drives have a read/write head for each platter surface. The heads float on a cushion of air generated by the spinning platter itself. Here we have a view of our RapidSpar unit performing a full disk image. The bad sectors shown in red are caused by loss of spindle RPM before the heads could retract to the ramp. The slowing spindle failed to make enough air cushion to keep the heads afloat. The heads fell on the platters and scraped away some of the data.