A data disk was a method of information storage using a rotating disk that could be accessed via a reading device.
In 2152, Captain Archer led a team to infiltrate a Suliban stealth cruiser and retrieve three data disks which contained proof that the Cabal had framed Enterprise for the destruction of the Paraagan II mining colony. As the data disks were not compatible with Starfleet technology, the crew had to construct an interface in order to access the information. The crew initially had difficulty in doing so, until they realized that all three disks had to be used in unison. (ENT: "Shockwave")
Two years later, the Enterprise crew recovered a data disk from Silik which contained the schematics to a temporal conduit that the Na'kuhl were attempting to build on Earth, in 1944 of an alternate timeline. (ENT: "Storm Front, Part II")
Later that year, Klingon data disks were used to store the Interspecies Medical Exchange's entire database. After the database was stolen by Klingons, the data disks were handed from Klingon Doctor Antaak to Denobulan Doctor Phlox. It was Antaak's intention that they use the data on them to determine if the IME had ever encountered the Klingon augment virus before. (ENT: "Affliction")
The Klingon data disks are not identified as such on screen; their designation as data disks comes from the final draft script of "Affliction".
Immediately after successfully leading a mutiny to take command of the ISS Enterprise (NX-01) in 2155 of the mirror universe, Jonathan Archer gave T'Pol a data disk which had the warp signature of a Tholian ship encoded on it. Under Archer's orders, the data disk was used by T'Pol to find the Tholian craft. (ENT: "In a Mirror, Darkly")
The final draft script of ENT: "In a Mirror, Darkly, Part II" consistently referred to a microtape in that installment as a data disk.