Hard Disk Logic Board

All modern hard disks are made with an intelligent circuit board integrated into the hard disk unit. Early hard disks were "dumb", meaning that virtually all of the control logic for controlling the hard disk itself was placed into the controller plugged into the PC; there were very little smarts on the drive itself, which had to be told specifically how to perform every action. This design meant that it was necessary for controllers to be very generalized; they could not be customized to the particulars of a given disk drive since they had to be able to work with any type. Older drives were similar enough, and sufficiently undemanding in terms of performance that this arrangement was acceptable. As newer drives were introduced with more features and faster speed, this approach became quite impractical, and once electronics miniaturization progressed far enough, it made sense to move most of the control functions to the drive itself.

The most common interface for PC hard disks is called IDE, which in fact stands for Integrated Drive Electronics. This name is something of a misnomer today. When it was introduced, IDE was distinguished from the other interfaces of the day by having the integrated electronics on the drive, instead of on the controller card plugged into the system bus like older interfaces. However, the term really refers to where the control logic is and not the interface itself, and since all hard disks today use integrated electronics the name doesn't mean anything any more, despite the fact that everyone continues to use it. The other popular PC hard disk interface today, SCSI, also uses drives that have integrated controllers. The more correct name for the IDE interface is AT Attachment or ATA; see here for more.

The logic board of a Cheetah 10,000 RPM 36 GB hard disk drive.
The main interface and power connectors are on the right-hand side;
auxiliary connectors on the bottom and left side. The bottom of the spindle
motor protrudes through a round hole made for it in the circuit board.

Today's hard disks contain logic boards that are in most ways more sophisticated than an entire early PC! In fact, most of them contain more memory and faster internal processors than an entire PC of even the mid-1980s. The logic board performs several important functions, and as hard disks become faster and more sophisticated, more functions are added to the logic board. This means the logic circuits need to be more powerful, to handle changes like geometry translation, advanced reliability features, more complicated head technologies, faster interfaces, and higher bandwidth data streaming from the disk itself.