Storage device including nonvolatile memory device and operating method of storage device

A storage device comprising:

• a nonvolatile memory device including a plurality of memory cells;
• and
• a controller configured to perform a write operation on at least one write unit included in the plurality of memory cells, and to perform an erase operation on at least one erase unit included in the plurality of memory cells,
• wherein the controller comprises an internal buffer,
• wherein when a backup operation on data stored in the internal buffer is required, the controller is configured to write the data stored in the internal buffer in a reserved erase unit,
• wherein after the backup operation, the controller is configured to migrate data stored in the reserved erase unit to target erase units, and
• wherein the controller is configured to select a first target erase unit to be first migrated among of the target erase units based on features of the target erase units.

A method for operating a storage device including a nonvolatile memory device and a controller, the method comprising:

• receiving, by the controller, a write request from an external host device;
• storing, by the controller, write data in an internal buffer, the write data being included in the write request;
• writing, by the controller, data stored in the internal buffer to a reserved erase unit, when a backup operation on the data stored in the internal buffer is required;
• and
• after the backup operation, migrating, by the controller, the data stored in the reserved erase unit to target erase units,
• wherein a first target erase unit to be first migrated among the target erase units is selected based on features of the target erase units.