According to one embodiment, a storage device comprises a nonvolatile memory, and a controller configured to perform a first data write operation in a first mode, and to perform a second data write operation in a second mode. Data of a first number of bits is written per memory cell in the first mode. Data of a second number of bits is written per memory cell in the second mode. The second number is larger than the first number. The controller reserves one or more free blocks as write destination block candidates of the first data write operation, perform the first data write operation for one of the write destination block candidates, and perform a garbage collection.