Hdd not recognized is caused by connection issues with the hardware components, outdated storage drivers, file system corruption, partition problems, or physical damage to the HDD. The issue with an external hard drive could be a result of a bad USB cable or low power supply. A loose connection of SATA or power cables for an internal hard drive could cause the same issue. Begin by trying another cable or port and checking the drive in Disk Management or BIOS. When the computer recognizes the drive but cannot access it, do not format it since there are chances that some vital files might be retrieved from it.