A double-sided removable hard disk with a capacity of 1 MB on each side. Univac published a brochure, according to which the Unidisc was intended for the UNIVAC 1004 and 1005 computers. I haven't found any photographs of the 1004 or 1005 systems with a Unidisc drive on the web, which leads me believe this was a prototype never released as a product.

A few other points:

The Unidisc is sometimes identified as a floppy disk, but, as stated in the brochure, it's definitely a hard disk. The disc is metal and the whole thing weighs 2.9 kg (6 lbs, 6 oz).

Although the date for the Unidisc is sometimes given as 1962, which is the year UNIVAC 1004 was released, the 1005 wasn't introduced until 1966. The Unidisc brochure mentions both the 1004 and 1005, which suggests the intended release was closer to 1966.

The Unidisc is sometimes identified as manufactured by Applied Magnetics Corporation under the name MDM-12, but according to an Applied Magnetics brochure, the MDM-12 was a fixed hard disk with up to four platters and a potential capacity of 16 MB.