A disk drive for encrypting user data. A motor configured to rotate a disk which stores encoded user data and an encryption flag which has not been encoded. An encoder/decoder processor configured to encode the user data that is written into the disk without encoding the encryption flag, and decodes the user data that is read out from the disk without decoding the encryption flag that is read out from the disk. An encryption processor configured to encrypt the user data at the encryption flag, wherein the encryption flag indicates encryption before the encoder/decoder processor starts encoding, and wherein the encryption processor obtains an encryption flag read out from the disk before the encoder/decoder processor completes decoding of the user data read out from the disk, commencing decryption of decoded data where the encryption flag indicates encryption before decoding of the user data is complete.