Disc Record explained:

Offset  Meaning
3       Density
        1       Single Density
        2       Double density
        4       Quad Density
        This is independant of bytes per sector.

A Disc Record may get updated in parts at different times when a disc is
being mounted:
Mount (fields which are filled in by ADFS/SCSIFS/other when the disc is
Mounted by miscellaneous reason code Mount):
  SectorSize
  SecsPerTrk
  Heads
  Density
  LowSector

StartUp (fields which are filled in by FileCore after the disc has been
identified as a FileCore disc using ServiceCall_IdentifyDisc):
  LinkBits
  BitSize
  RASkew
  BootOpt
  Zones
  ZoneSpare
  RootDir

Identify (fields which are filled in by, or modified by FileCore, or
whatever MultiFS filing system which has identified the disc):
  Heads                 (when Sequence sides is altered, Heads must be
                        altered in step with it)
  SequenceSides
  DoubleStep
  DiscSize
  DiscId
  DiscName


When FileCore tries to match a mounted, positively identified disc disc
against those which it has already seen it bases its comparison on these
fields:
SectorSize
SecsPerTrk
Heads
LowSector
SequenceSides
DoubleStep
DiscId
DiscName
These are the fields which get filled in by Mount and Identify.
