the inability to "read" some sectors is not always an indication that a drive is about to fail. one way that unreadable sectors may be created, even when the drive is functioning within specification, is through a sudden power failure while the drive is writing. in order to prevent this problem, modern hard drives will always finish writing at least the current sector immediately after the power fails (typically using rotational energy from the disk). also, even if the physical disk is damaged at one location, such that a certain sector is unreadable, the disk may be able to use spare space to replace the bad area, so that the sector can be overwritten.