Patent ID: 8760921

Claim:
A storage device comprising: a nonvolatile memory including blocks, each of the blocks including pages each constituted by a memory cell group, each memory cell being capable of being set at a first threshold voltage and a second threshold voltage; a controller configured to copy data stored in a first page in a first block to a second page in a second block; and an ECC circuit configured to correct an error in data read from the nonvolatile memory, wherein the controller reads data from a part of the first page by using an upper limit voltage and lower limit voltage for determining a threshold voltage of a memory cell, performs a direct copy operation in the nonvolatile memory without via the ECC circuit if the number of error cells having threshold voltages higher than the lower limit voltage and lower than or equal to the upper limit voltage is less than or equal to a specified value, and performs error correction by using the ECC circuit if the number of error cells exceeds the specified value.