MFM disk geometry

[Daniel Malament]

I'm trying to pull data off an old MFM HD, and I've gotten to the point 
where the only obstacle is disk geometry.  I have a P3 machine which 
will disable the primary IDE controller in favor of the MFM controller, 
but boot off of an OpenBSD disk on the secondary IDE.  OpenBSD sees the 
MFM disk just fine, but gives it the wrong CHS, which wouldn't matter 
except that it's evidently too old to do LBA, since OpenBSD is using CHS 
mode.  I can pull the first few sectors off of the disk, but then I get 
errors I'm guessing are because of the geometry mismatch.

Is there any way at all to change the CHS values the kernel is using for 
a disk?  fdisk with -chs doesn't seem to produce a permanent change (I 
guess the values are just used for calculating?), and the 
machdep.bios.etc sysctls are read-only.  Google and the archives haven't 
turned up anything terribly useful, although it sounds like what I'm 
trying to do may not be possible.  If not, anyone have any alternate 
suggestions?

Incidentally, I have a bunch of other old crap around, but my efforts to 
get everything working on a machine that will let me set the CHS in the 
BIOS haven't gotten anywhere yet...