MFM vs RLL hard drives

The most common type of hard drive for most of the 1980s was called MFM. At least that’s what we commonly call them today. That was to distinguish them from a similar type we called RLL. How much difference there was between the two was always a bit controversial. But let’s look at MFM vs RLL hard drives.

The ST-506 interface

Both MFM and RLL hard drives had the same type of connector to interface to whatever computer you were using it with. You can think of that interface, ST-506, as an extended version of the floppy disk interface. It used two connectors, and one of them was 34 pins like a floppy cable. The second 20 pin connector was exclusively for data.

Unlike newer drive types like IDE, both MFM and RLL drives needed a fairly complex and expensive controller board. The electronics on the drive itself just controlled the spindle motor and the stepper motor. The actual work of encoding and decoding the data so that the computer’s operating system could use the drive took place on the controller board.

Using a more sophisticated controller board meant you could get more out of the disk hardware.

RLL encoding vs MFM

The difference between MFM and RLL was the encoding mechanism. MFM stands for modified frequency modulation and was a standard inherited from floppy disks. MFM was less efficient than RLL. RLL could store 50% more data on the same media.

Technically, MFM is a type of run length limited encoding, so we can think of RLL as a more efficient relative of MFM.

If you wanted to upgrade your storage and you went to replace and existing MFM drive with an RLL drive, you were in for a disappointment if you didn’t also replace your controller. With some fiddling, you could make the drive work, but you would lose 1/3 of its capacity.

The cheap upgrade to get 50% more storage out of MFM drives

The reverse was also true, and it wasn’t exactly a secret. You could keep your existing drive, replace the controller, and with a bit of fiddling, increase your capacity 50%. And if you were buying a drive and controller new, you could save some money by buying a cheaper MFM drive and an RLL controller, essentially getting 60 MB for the cost of 40.

Drive manufacturers said at the time that RLL drives were built to stricter tolerances so they could work reliably at the larger capacity. If you used an RLL controller with an MFM drive and anything went wrong, they would blame it on the controller. Much like overclocking, it would work, but you would void your warranty.

The other thing they would tell you was that when they made a batch of drives, the best drives were certified for RLL, and RLL drives that didn’t make the cut might work reliably as MFM drives.

I never heard of anyone trying this and having any problems. But I never tried it myself either. I did briefly own a couple of MFM drives, but by the time I got into PCs, IDE had become the standard.

The controversy

Much like buying single-sided floppies and formatting them as double-sided, or buying double density 3.5″ floppy disks and drilling a hole in them to turn them into high density disks, it was a controversial practice to save money. The people who did it and didn’t have any problems were very outspoken about it. Risk-averse people were sometimes very outspoken about why all three of these practices were a bad idea.

I wouldn’t say any of the debates wherever settled conclusively either. The affected technologies just went obsolete and arguing about it became a moot point. But it made for some spirited discussions on BBSs in the 1980s.