transfer rates made them attractive for high-performance needs. A typical use was overlay linkage (e.g. for OS and application subroutines) for program sections written to alternate in the same memory regions. Fixed head disks and drums were particularly effective as paging devices on the early virtual memory systems. The 2305, although often called a "drum" was actually a head-per-track disk device, with 12 recording surfaces and a data transfer rate up to 3 MB per second. Rarely seen was the IBM 2321 Data Cell, a mechanically complex device that contained multiple magnetic strips to hold data; strips could be randomly accessed, placed upon a cylinder-shaped drum for read/write operations; then returned to an internal storage cartridge. The IBM Data Cell [noodle picker] was among several IBM trademarked "speedy" mass online direct-access storage peripherals (reincarnated in recent years as "virtual tape" and automated tape librarian peripherals). The 2321 file had a capacity of 400 MB, at the time when the 2311 disk drive only had 7.2 MB. The IBM Data Cell was proposed to fill cost/capacity/speed gap between magnetic tapes—which had high capacity with relatively low cost per stored byte—and disks, which had higher expense per byte. Some installations also found the electromechanical operation less dependable and opted for less mechanical forms of direct-access storage. The Model 44 was unique in offering an integrated single-disk drive as a standard feature. This drive used the 2315 "ramkit" cartridge and provided 1,171,200 bytes of storage. Tape drives The 2400-series of 1/2" magnetic tape units consisted of the 2401 and 2402 Models 1-6 Magnetic Tape Units, the 2403 Models 1-6 Magnetic Tape Unit and Control, the 2404 Models 1-3 Magnetic Tape Unit and Control, and the 2803/2804 Models 1 and 2 Tape Control Units. The later 2415 Magnetic Tape Unit and Control, introduced in 1967 contained two, four, or six tape drives and a control in a single unit, and was slower and cheaper. The 2415 drives and control were not marketed separately. With System/360, IBM switched from IBM 7-track to 9-track tape format. Some 2400-series drives could be purchased that read and wrote 7-track tapes for compatibility with the older IBM 729 tape drives. In 1968, the IBM 2420 tape system was released, offering much higher data rates, self-threading tape operation and 1600bpi packing density. It remained in the product line until 1979. Unit record devices Punched card devices included the 2501 card