A Hard Disk Drive (HDD) is a data storage device using magnetic storage on rotating platters to read and write digital information. HDDs have been the primary mass storage technology for decades, offering high capacity at relatively low cost. They contain mechanical components—spinning disks and moving read/write heads—making them susceptible to damage from shock or vibration and limiting their speed. HDDs are increasingly being replaced by Solid State Drives (SSDs), which use flash memory with no moving parts, offering faster performance, greater durability, and lower power consumption, though at higher cost per gigabyte. For government IT systems, storage choices affect performance, reliability, cost, and data center energy consumption. Many organizations use HDDs for bulk storage and archives where speed is less critical, while SSDs serve applications requiring fast data access. Understanding storage technology helps evaluate IT decisions and investments.