See also history of IBM magnetic disk drives. If the files are needed, they are migrated back to magnetic disk. Type fonts were stored digitally on conventional magnetic disk drives. Magnetic disk storage became available in 1966. The ODR was a large-volume storage device that used an optical magnetic disk system. The first 16 terabyte magnetic disk drives were delivered by mid 2019. The magnetic disk drive symbolized a monumental advance in mass-storage technology. Magnetic disk heads and magnetic tape heads cannot pass DC (direct current).