Old Disc Drives - MFM Hard Disk Drives

Synopsis

Modified frequency modulation -- a very old data encoding standard. These units are part of the ST-506 or ST-412 class of drives. The two ribbon cable host interface is also called ST-506 (ST-412). But I always erroneously called these units MFM drives and their host interface cards MFM controllers.

MiniScribe 8425

I pulled this disk drive out of a 286 machine that was going to be sold to the antiquity shop five streets down from my place, about five years ago (that should be 1999). Unluckily, the drive was damaged and I think I made things worse when I rotated the shaft of the stepper motor. Although there is a label affixed to the drive saying: "DO NOT ROTATE INTERRUPTER"), my curiosity went strong on this one. I did this because the disk drive wouldn't start. Probably the heads were stuck to the platters. After rotating the interrupter about 3 mm, the disk drive powered on and did some never-ending tests. I presumed the drive was in factory test mode because the stepper motor was making an infernal noise and wouldn't stop spinning back and forth. After a couple of minutes, the drive self powered off and since then I couldn't power it back on.

My drive is dated 31 OCT 1989 and I think it is the oldest one I have by far.

It is finished quite well, enclosed in a beautiful cover but MiniScribe used the weird hexagonal (torx) bolts as Seagate did with some of its disk drives. This made opening more difficult than I'd wanted but I finally managed to get the top cover out and, as you can see in the picture, the mechanism relates to what you can find in floppy drive. I should also say that MiniScribe was Seagate's main competitor.

As you can see in the picture, on the top cover (behind the disk drive) there is the list of "AS SHIPPED DEFECT CRITERIA". It is full of factory-marked defects. Sector 0 is error-free, as a general rule. Also notice the weird color of the thin film platters. It looks like oxide but it is clearly not. Finally, notice the big black silver and gold, external-mounted stepper motor. The internal shaft of the motor is connected to a cam that drives the actuator.

You could not miss the way the LED is mounted. It is inserted in a retaining connector from which you can easily remove it. Also, if you turn the disk upside down to see the components, you will not find a single SMD part, as Seagate used for the majority of disk drives of the era. This logic board is entirely built with standard THD parts.

TECHNICAL DATA

CAPACITY: 21.4 Mb
ACTUATOR TYPE: STEPPER
DISCS: 2
MEDIA TYPE: THIN FILM
RECORDING METHOD: MFM 1,3
SPINDLE SPEED: 3.600 rpm
INTERFACE: ST-506 (MFM)
AVERAGE ACCESS: 68 ms

Seagate ST-251

I have acquired this disk drive from a friend, in 2003, if I recall correctly. As you can see from the picture, it has a similar design with the Seagate ST-238R RLL disk drive. Same design may be found on Seagate ST-225 and other similar Seagate drives of the era. This drive has affixed the FACTORY REPAIRED label which means that it was repaired at least one time before it arrived to me. As like almost all MFM disk drives, this model has the defects list. As always, sector 0 is error-free.

The Seagate ST-251 disk drives were slow and very noisy. When you power on one of these drives, it takes about 4 to 5 seconds for the platters to reach maximum speed while they make this infernal vacuum cleaner like noise. But they'd proved reliable and had a long life span.

I haven't managed to test this disk further on to put my impressions here.

TECHNICAL DATA

CAPACITY: 42.8 Mb
ACTUATOR TYPE: STEPPER
DISCS: 3
MEDIA TYPE: THIN FILM
RECORDING METHOD: MFM 1,3
SPINDLE SPEED: 3.600 rpm
INTERFACE: ST-412 (MFM)
AVERAGE ACCESS: 38 ms

Thank you for spending your time reading these old pages.