HDD, or Hard Drive Divinity, is an ability used by the goddesses of Gamindustri which allows them to transform and access their full power. There are some noticeable differences between the goddesses and their respective human forms, though what specifically varies for each goddess. Besides the obvious visual change, these may include personality changes. The exact stats of this form can be customized with Processor Units.
In Hyperdimension Neptunia mk2, the CPU candidates have two versions of this form, with the second form being obtained via the power of the mascot characters. Aside from granting them a different appearance, it also grants each one a new move.