Hard disks: if you read this, it's pretty much certain you use one or more of the things. They're pretty simple: they basically present a bunch of 512-byte sectors, numbered by an increasing address, also known as the LBA or Logical Block Address. The PC the HD is connected to can read or write data to and from these sectors. Usually, a file system is used that abstracts all those sectors to files and folders.

If you look at an HD from that naive standpoint, you would think the hardware should be pretty simple: all you need is something that connects to a SATA-port which can then position the read/write-head and read or write data from or to the platters. But maybe more is involved: don't hard disks also handle bad block management and SMART attributes, and don't they usually have some cache they must somehow manage?

All that implies there's some intelligence in an hard disk, and intelligence usually implies hackability.