According to one embodiment, a tiered storage system includes a tiered storage device and a computer. The computer uses the tiered storage device, and includes a file system and a correction support unit. If an access request from an application is a write request to request overwriting of data, the file system executes a copy-on-write operation. The correction support unit causes the storage controller to carry over an access count manacled by the storage controller and associated with the logical block address of a copy source in the copy-on-write operation, to an access count associated with the logical block address of a copy destination in the copy-on-write operation.