fixed disk unit; immovable disk drive
Computing term for a non-removable disk storage device, such as a hard disk drive built into a computer.
This server is equipped with a large-capacity fixed disk unit.
ハードディスク is the common term for a hard disk drive, while 固定ディスク装置 is a more formal technical term emphasizing that the disk is not removable.
Compound of 固定 (fixed) + ディスク (disk) + 装置 (device, unit). A straightforward technical term.