A nonvolatile storage device in accordance with the inventive concepts includes a nonvolatile memory device comprising a first memory area, a second memory area, and a memory controller. The memory controller includes a first register configured to store reliable mode information, and a second register configured to store operating system (OS) image information. The memory controller is configured to receive a command from a host based on the reliable mode information; determine whether the command is a write request for an OS image and whether OS image information accompanying the command matches the OS image information stored in the second register; write the OS image to the first memory area if the OS image information accompanying the command matches the OS image information stored in the second register, and block data migration of the OS image from the first memory area to the second memory area.