A hard disk drive is a device that can store information in a non-volatile way. This means that when a computer is powered off, the content on a hard disk drive is retained. For the purpose of this discussion, we will also count flash drives (commonly known as “thumb drives”) and solid-state disks (SSDs) as hard disk drives.
From the software perspective, a hard disk drive is really not much more than a huge array of sectors. The hardware sector size is fixed unless the firmware of a hard disk drive is changed. The older standard is 512 bytes. However, as the capacity of hard disk drives increase, the 512B sector size becomes inefficient. As a result, new hard disk drives come with a sector size of 4kB (4096 bytes).
On a PC (using PC BIOS, basic input/output system), some sectors of a hard disk drive are special.