MO drives usually have a DIP switch to set the device type mode. Depending on how this is set, the drive reports itself as a direct access device (type 0, the normal value for hard disks), or as an optical memory device (type 7, the normal value for optical disks). In the absence of problems, it is probably best to set the device type to optical memory.
Some programs, especially those that access SCSI drives directly, may check the device type and refuse to work with drives of type other than zero. Commodore's HDToolBox is an example of this, though I have developed a patch to get around this problem. Some SCSI drivers might refuse to boot from non-type-0 devices. My old GVP controller does not have this problem.
When my drive is set to optical mode ShapeShifter refuses to recognise it; the emulation gets stuck before booting. This may be due to bugs in the old version of gvpscsi.device that I use, rather than ShapeShifter though.
Apart from the reported device type, in direct access mode two hard disk mode pages are returned, which are not returned in optical memory mode. They are:
The values for number of cylinders, heads and sectors per track given in these two pages are "fake", for compatibility with PCs. For example, for 640MB disks, the geometry values reported in the device format and rigid disk parameter pages are 606 cylinders, 8 heads and 64 sectors per track, which implies that the total number of sectors is 606×8×64 = 310272. The actual number of sectors is 310352.
Some hard disk partitioning programs may use the 606×8×64 value instead of working out values based on the total disk size.