On most models Hard Drives are not included and must be purchased separately. Note, most Network Attached Storage needs clean drives and will not read NTFS or FAT file systems because these units use their own file allocation table (to escape licensing fees and to make easier circuitry) but some will do Linux file tables. Most likely files written to Network Attached Storage will not be read by a computer if you were to attach the hard drive directly to a computer (in case Network Attached Storage unit breaks down). If the unit does break down, usually, exactly the same model can be utilized with the survived hard drives to retrieve the data. Therefore, it is a good idea to buy two Network Attached Storage units and keep one just in case a recovery will be needed. A few years from now these models might not be widely available and chance to find one would be slim. How valuable your data to you?