A hard drive was a piece of computing equipment that could store information.
With permission from the Afghan government, CTU raided Syed Ali's safe house, and among the items found was a well-concealed hard drive encrypted with a Z cube function. A team of analysts broke the encryption, and on it found an audio recording that supposedly implicated three Middle Eastern heads of state in a plot to nuke Los Angeles. ("Day 2: 10:00pm-11:00pm")
During Day 2, Eve went to Alex Hewitt's apartment and took lots of his equipment and files. However, she failed to find a hard drive hidden beneath one of Hewitt's drawer, which he retrieved when Jack Bauer and Sherry Palmer found him. The drive contained a tarball of his audio software as well as recording of many phone conversations. ("Day 2: 5:00am-6:00am")
A CTU hard drive was stolen by Peter Madsen and Joseph Sin-Chung after they broke into CTU Los Angeles. Kim Bauer attempted to encrypt the data to stop them downloading it, but when Madsen executed Sean Walker she handed the drive over, with a tracking device attached. Some of the data on the drive was concerning undercover agents, but it was later recovered by Agent Chase Edmunds. (The Game · "To The Roof", "Escaping Joseph Sin-Chung's Base")
On Day 4, Habib Marwan dispatched Nicole to Mitch Anderson's apartment to search for a hard drive containing incriminating information. Nicole, posing as an FBI agent, was soon greeted by Jack Bauer and Agent Hart, who were also in search of Anderson. While searching the apartment, Hart discovered a secret compartment behind a false wall, which contained the hard drive. Nicole then discreetly killed Hart and attempted to take out Bauer, but was killed herself. Jack then hooked the hard drive up to a laptop and, with Edgar Stiles' help, accessed its contents. On it were a flight simulator and schematics for an F-117 Nighthawk. ("Day 4: 10:00pm-11:00pm")
During Day 9, after Naveed Shabazz was forced to pilot the stolen drones by Margot Al-Harazi, he hid a hard disk drive of evidence under the floorboards of his and his wife Simone's room. Naveed was executed for betrayal, but Simone did not tell her mother about the existence of the disk. After she was badly injured and taken to the CIA London station, Simone was questioned by Kate Morgan for the location of her mother. She gave up the location of her mother's house and told Kate about Naveed's disk, suggesting that it might contain a way into Margot's system. ("Day 9: 4:00pm-5:00pm")
Kate sent a tactical team to Margot's house which they found abandoned. The team was able to locate Naveed's disk and uploaded its contents to the CIA station. Kate then ordered Gavin Leonard to send the data to Chloe O'Brian, who was also attempting to stop Margot. Chloe was able to use the information on the disk to hack into Margot's system, which ultimately allowed her to hack the drone's camera feed and save President Heller. She later used the information to track Margot down to Tundel House. ("Day 9: 6:00pm-7:00pm", "7:00pm-8:00pm")