# Reading an MFM hard drive with a floppy disc analyser

Philip Pemberton classiccmp at philpem.me.uk
Fri Dec 5 04:12:44 CST 2008

```I've just been thinking about the problem of imaging/copying MFM hard discs...
I've run the numbers, and this seems right to me, but I'd appreciate a sanity
check.

Let's say you have an ST-412 drive you want to read. You've also got a
Catweasel or something like it, that has somehow had an extra bunch of I/O
lines added to it (to control the head select lines, etc. on the HDD).

An ST-412 rotates at 3600 RPM. That's (3600/60)=60 revolutions per second, or
1/60 = 16.66(recurring) milliseconds per revolution.

The drive's data rate is 5 megabits per second, but could be lower (or indeed
higher). But the spec says 5Mbps, and for the sake of argument I'm going to
stick with that...

(5Mbps/1000) = 5Kbits per millisecond.

5Kbits * 16.667 = 83.35 kilobits per track, absolute maximum.

A Catweasel records the data from a disc by measuring the time between flux
transitions.

If we assume every data bit on the disc is a "1" (MFM coding "01") then you'd
need to store at most 90,000 timing samples per disc track.

worth of buffer RAM, and enough I/O lines to drive the HDD, you could read an
MFM hard drive track-by-track and copy it onto another drive of the same type?

Do my calculations look right?
With 304 cylinders and 4 heads, that works out to 304 * 4 * 83.35 = 101353.6
Kbits, which does seem awfully low to me...

Thanks,
--
Phil.
classiccmp at philpem.me.uk
http://www.philpem.me.uk/

```