Actuator - Also known as a HDA (Head Disk Assembly), it is a device containing the physical disk platters and the heads that access them in a sealed enclosure to protect against contamination. The mechanism which moves the heads in and out to a specific track are external to the enclosure. This arrangement was developed to solve the failures caused by particulate contamination on early disk drives where the disk platters were (at least when changing packs) open to the ambient air in the computer room. The new arrangement significantly increased the reliability of early disk drives.
An old style disk assembly inside it's travel cover which is removed when mounting the pack on the drive spindle. While the cover is off the magnetic surface of the disks is subject to contamination.
An HDA assembly with it's top cover off. The drive motor for the spindle was not inside the case, but below it. Often the mechanism which moves the heads is located outside of the case.