【Mitsubishi Electric】 MR521 Hard Disk Unit

Announced by Mitsubishi Electric in 1983, this was a sealed hard disk unit which used a 5.25-inch (13.3cm) magnetic disk, and halved the thickness of its previous model. Its maximum memory capacity was 12.75MB.

This unit had the following features:

  • Thickness was half that (half size: 41mm) of its previous model (M4863), and this enabled twice the packaging density.
  • Even though it was half size, the unit achieved a high memory capacity of 12.75MB.
  • It achieved high-speed transfer at 5Mbit/s, with high-speed access at an average seek time of 85ms.
  • A simple, high-precision, high-reliability mechanism was realized using a positioning mechanism employing a stepping motor, and a direct spindle mechanism employing a DC brushless motor.
  • The structure was completely sealed, so the unit was not affected by adverse dust environments.
  • It used an industry-standard ST506 interface.
Main Specifications of MR521 Hard Disk Unit
Unit memory capacity 12.75MB
Side capacity 6.37MB
Track capacity 10,416byte
Number of disks 1
Recordable sides 2
Number of cylinders 612
Recording system MFM
Bit density 9,201bit/inch
Track density 690track/inch
Data transfer speed 5.0Mbit/s(625Kbyte/s)
Positioning time Min. 18ms,Max. 199ms,Average 85ms
Rotation speed 3,536rpm
Rotational delay Average 8.48ms
External dimensions(Width x Height x Depth) 146mm×41.3mm×203.2mm
Weight 1.6kg

In 1984, Mitsubishi announced the MR522 which achieved a memory capacity of 25.5MB by incorporating two recording disks (4 recording capable sides) in a cabinet of the same size.
In 1985, Mitsubishi speeded up the average positioning time to 38ms by using a miniaturized voice coil motor, increased the track density by 1.5 times to 1,028track/inch, and set the number of cylinders to 971. They announced the MR533 with 2 disks (3 memory capable sides) and a memory capacity of 30.33MB, and the MR535 with 3 disks (5 memory capable sides) and a memory capacity of 50.55MB.

MR521 Hard Disk Unit