A 3.5-inch hard drive (HDD) is a hard drive that you can use to upgrade your desktop. Install a 3.5-inch hard drive in your desktop PC for a larger storage capacity or faster performance. This allows you to save more files and transfer files from and to the drive faster. You can also easily connect the 3.5-inch drive to your NAS. Once built-in, you'll be able to save files for a while again.