<I've forgotten the rule on this exactly, but you can use a RLL on a MFM ST5
<
<RLL is a form of MFM.  The interface is compatable but there are some gotch
<
<1) I think you have to assume there are 1/2 as many sectors per track.

no.  That would be dependant on the controller and the sector size.  Often 
for a fixed sector size the RLL encoded one will have more sectors.

<2) You have to re-low-level format the drive.

It's a must going either way.  Also only format the drive when it's reached 
operating temperature.  It does make a difference.

<3) Not all drives will tolerate the change.

Some are not up to doing reliable RLL.  Interrelated with controller.  
Also those that don't do rll well often have a poor MFM record or are 
very old drives.

<4) I'm going on 7 year old memories and can't be trusted :-)

Not a disk problem. ;)

There are some I'd add, some controllers do RLL better than others with 
the same drive.  Also short cables and correct termination is an absolute 
must. I've found RD53s(micropolus 1325) do RLL very well as do the RD52s 
(Quantum D540).  St225s are ok, ST238s are 225s that are certified RLL but, 
ST251s tend to be very temperature sensitive and get flakey.

Allison