Old MFM/RLL drives with stepper positioners generally have no servo. The same can't be
said of voice-coil positioned drives; they could use any means, including hardware optical
servos, etc. but often used a surface and a head for that purpose... so yes, you can
render one of these drives useless with a magnet if yours has a servo surface. (This is
not the same as embedded servo, which places servo information -with- the data on the same
surfaces.)