Yes, /drive1 is /dev/hdc4 on a MkI too. Actually it's on the MkI that this is "same as on an ordinary PC". You relabel the devices for the first IDE channels slave drive to /dev/hdcX on the MkII (where it would "normally" be called /dev/hdbX), I guess so that it can use the same code/same instructions as the MkI...

/Michael