According to an embodiment, a storage controller comprises a circuitry configured to implement an address generator, a reader, and a duplication detector. The address generator is configured to generate a scan address indicating each storage area of a storage that stores therein externally written data, according to a particular scan pattern for defining an order of an address of data to be read. The reader is configured to read data from the storage area of the storage indicated by the scan address. The duplication detector is configured to detect whether the data read by the reader is a duplicate of any one of a past predetermined number of pieces of data.