A hard drive was a data storage device. WOTAN was uploaded onto a hard drive from a floppy disk by Torchwood One. The hard drive was later wiped and thrown out, but it was found by Julian Delaware who was able to recover WOTAN. (AUDIO: The Law Machines)
Gallifreyan camera drones had hard drives with polycarbide layering. As a result, Narvin was able to recover images from one of Commentator Antimon's hard drives, allowing him and Leela to identify him as the one who bombed the Gallifreyan Academy. (AUDIO: Imperiatrix)