MFM Hard Drive on Pentium II PC

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The “older” PCs that I have laying around in the workshop all tend to be Pentium II era machines or later. Unfortunately I don’t have much in the way of working 8086, 286, 386 or 486 PCs.

The Pentium 2 machines work just fine for me for almost all of the older hardware and MSDOS programs that I still need to use occasionally, such as EPROM programmers, PUTR, ImageDisk, etc.

One area that causes me trouble however is attempting to hook up old MFM hard drives (such as a 20MB Seagate ST-225) to one of my older PCs, when I need to attempt to recover files from such a drive, or run diagnostics on it, or attempt to low-level format it. So on this page I have documented what works (or not) and the limited degree of success that I have had.

Shown to the right here is an ST-225 (re-badged by DEC as an RD31) and its Western Digital MFM controller model WD1003-WAH, as removed from a DEC Vaxmate in January 2017.

I will say at the outset that if you have an earlier PC (such as a 386 system) that does NOT have an onboard IDE controller, I suspect that you will have far less difficulty in getting your MFM drive and controller up and running. So please consider that path, if it is available to you.

The hardware I am using for the purpose of this article is as follows:

  • A Pentium II motherboard with one ISA slot running “Award Software” BIOS showing “(C) 1994 – 1998”
  • 1 x Seagate ST-225 MFM hard drive
  • 1 x Western Digital WD1003-WAH ISA MFM controller board
  • “Straight-through” 20-way and 34-way cables between the controller board and the MFM hard drive

Configuring the MFM stuff

Here are the basic steps that you need to take, to hook up an ST-225 to a Pentium 2 motherboard:

Firstly, go into the PC’s BIOS settings (usually accessed by pressing F2 when the PC is booting up) and do the following:

  • Disable the onboard IDE controller
  • Disable the onboard floppy controller, if the MFM controller board that you will be using (for the MFM hard drive) has its own onboard floppy controller
  • Change the drive settings for the “IDE Primary Master” drive. You will typically have to specify “Manual” (rather than “Auto”) here. Then enter the Cylinder, Head and Sector details for the drive you will be using. For the ST-225, these figures are: 615, 4, 17. Importantly, make sure you change “Access Mode” from “LBA” to “Normal”. If you fail to do this step, the BIOS will pass incorrect drive geometry details to programs attempting to access the MFM drive. For example, for the ST-225, it tells SpeedStor that the drive has 20 Cylinders, 32 tracks and 63 sectors.

Now plug your MFM controller board into the PC’s motherboard. The controller board probably has an ISA edge connector. So you’re going to need a motherboard that is old enough to have at least one ISA slot available.

Now plug the MFM drive into the controller. There will be a 20-way and 34-way cable. The 20-way cable will be a “straight through” cable. The 34-way cable will either be “straight through” or will have pins 25-29 “flipped” at the drive end. Note that if your cable has pins 10-16 “flipped” it is NOT the right cable and must NOT be used with an MFM hard drive (it is a floppy drive cable). For simplicity, I won’t comment on the “flipped” cable here other than to say that it affects the “Drive Select” jumper that you need to have in place on the MFM drive itself. Check your controller board carefully. It will likely have 2 x 20-way connectors on it (for the first and second MFM hard drive respectively), and it may have 2 x 34-way connectors on it (one for a daisy-chain cable to the two MFM hard drives, and the other for your floppy drives). If the board is not clearly marked, you will need to consult the documentation for the board to make sure you are using the correct connectors.

On the back of the ST-225 (between the 20-way and 34-way edge connectors) you’ll find an 8×2 array of jumper pins. Assuming you are using an MFM drive attached to “straight through” cables, make sure you have the “DS1″ jumper installed and the other 7 jumpers removed. The DS1 jumper is in the position closest to the 20-way edge connector.

Make sure the drive has a terminating resistor pack fitted. If you later expand your system to include 2 x MFM hard drives, make sure there is only one resistor pack fitted, and that it is in the drive located the furthest from the controller on the 34-way cable.

Now connect the power connector. This will be a standard 4-pin connector as used universally on 5.25” floppy drives.

Booting it up

If your drive has a working installation of MSDOS on it and you are exceedingly lucky, your drive may now be capable of booting your Pentium 2 system. This has only worked for me once. I suspect part of the problem is that an MFM drive that was formatted and written on one controller board will often not be readable at all with a different controller board. A second major reason for problems these days is that MFM drives are getting to be very unreliable.

Running diagnostics on the drive

It seems there are a few MSDOS utilities that were useful “back in the day” to perform non-destructive seek and read tests on an MFM hard drive.

The two that I have played with are: SpinRite II and SpeedStor.

I’ve not had any luck getting SpinRite II to run on my Pentium II machine. All looks fine until you select an option from the main menu, such as “Quick Surface Scan”. Then it reports an error saying “SpinRite has not found a compatible hard disk sub-system in this computer system, the chosen device’s sector zero is unreadable, or the device is unreadable to SpinRite”. I get this error message from SpinRite even when SpeedStor works perfectly well on the same system.

I have had good results using SpeedStor on my Pentium II machine. Provided you have put the CHS values in the BIOS settings page, and disabled LBA mode (as described above), SpeedStor works well. You can use it to test the controller, and perform non-destructive seek and read tests. More information about SpeedStor (and download links) are available at

It appears that SpinRite may still be available for purchase from Gibson Research Corporation. You may want to check with them, before purchasing, that the current version still supports MFM hard drives on ISA controller boards.

Formatting an MFM hard drive

An MFM drive must be low-level formatted before the MSDOS routines are used to partition and high-level format the drive.

Some MFM controller boards have on-board BIOS routines that perform this function. If your controller has built-in low-level formatting routines, that is what you should use. Typically these routines are accessed by using the DOS DEBUG command to start the formatting routine in the controller’s BIOS. You will need to consult the documentation for the controller board for further information about this.

But many controllers do not have an on-board low-level formatting routing. The WD1003-WAH is an example of a controller board that does NOT have an on-board low-level formatting routine.

Your options then are to use one of the following:

Some useful links on low-level formatting:

Remember that once you have completed the low-level format of the MFM hard drive, you need to use the DOS FDISK command to partition the drive, and then the DOS FORMAT command to high-level format each partition.

Other tips

Because you will likely have your PC’s IDE controller disabled when playing around with the MFM drive, it is helpful to make up a bootable floppy disk (1.2MB 5.25″ or 1.44MB 3.5″) that has the utilities on it that you will need. I typically make up a bootable MSDOS 6.22 disk with SppedStor on it, as well as the MSDOS utilities DEBUG, FDISK and FORMAT.

If you cannot get your MFM drive to work properly (or at all) it may very well be faulty. Probably 70% of the MFM drives that I have tried to work with over the past few years have either failed totally, or have an unacceptably high number of defective tracks. Warning signs that your drive is at end of life include: drive gets very hot after 30-60 minutes of idling, drive spins down rapidly when power is removed, drive won’t spin up unless the casing is yawed or bumped, or drive RPM can be heard to fluctuate.

Useful documents


Here are some additional links to useful information about MFM hard drives:

Revision History

7 January 2017: Posted initial version.