Apparatuses and methods for defect replacement when an optical storage medium is read are provided. When the defect management is LOW, a pick-up head retrieves a set of data from the optical storage medium; a defect detector detects whether there is a defect in the set; if yes, a processor determines whether a replacement for the defect is in the set; and if yes, an interface transmits the replacement from the set. When the defect management is CRD, a buffer temporarily stores data retrieved from the optical storage medium; a defect detector detects whether there is a set of defects in the data; if yes, a comparator compares a length of the set of defects with a defect threshold length; a pick-up head reads more data from the optical storage medium continuously until the buffer reaches a buffer threshold if the length is compared shorter than the defect threshold length; and the pick-up head reads a set of replacements for the set of defects directly if the length is compared longer than the defect threshold length.