An external-pump type delivery device for delivering medication or other fluidic media to the body of a user-patient, the delivery device comprising: a first housing portion adapted to be secured to a user; a second housing portion configured to be selectively engaged with and disengaged from the first housing portion, one of the first housing portion and the second housing portion for supporting a reservoir for containing fluidic media; a drive device configured to be operatively engagable with a plunger arm connected to a plunger head arranged for movement by the drive device in an axial direction of the reservoir, the drive device configured to drive fluidic media from the reservoir in a case where the drive device is operatively engaged with the reservoir and the plunger arm is moved in the axial direction relative to the reservoir; and a drive linkage for operatively engaging the drive device with the plunger arm to allow the drive device to move the plunger arm, the drive linkage including a first gear (174, 172) and a second gear operatively engaged with each other, the first gear (174, 172) engageable with the plunger arm; the second gear (176, 178) supported on a shaft having a longitudinal dimension, characterised in that the second gear (176, 178) configured to be movable along the longitudinal dimension of the shaft at least between a first position and a second position to provide a range of rotational motion of the first gear (174, 172) to allow the first gear to engage the plunger arm; and the second gear (176, 178) configured to be moveable to the first position to reduce the range of rotational motion of the first gear (174, 172) relative to the plunger arm in a case where the first gear (174, 172) and the plunger arm are engaged and the second gear (176, 178) is in the second position. a first housing portion adapted to be secured to a user; a second housing portion configured to be selectively engaged with and disengaged from the first housing portion, one of the first housing portion and the second housing portion for supporting a reservoir for containing fluidic media; a drive device configured to be operatively engagable with a plunger arm connected to a plunger head arranged for movement by the drive device in an axial direction of the reservoir, the drive device configured to drive fluidic media from the reservoir in a case where the drive device is operatively engaged with the reservoir and the plunger arm is moved in the axial direction relative to the reservoir; and a drive linkage for operatively engaging the drive device with the plunger arm to allow the drive device to move the plunger arm, the drive linkage including a first gear (174, 172) and a second gear operatively engaged with each other, the first gear (174, 172) engageable with the plunger arm; the second gear (176, 178) supported on a shaft having a longitudinal dimension, characterised in that the second gear (176, 178) configured to be movable along the longitudinal dimension of the shaft at least between a first position and a second position to provide a range of rotational motion of the first gear (174, 172) to allow the first gear to engage the plunger arm; and the second gear (176, 178) configured to be moveable to the first position to reduce the range of rotational motion of the first gear (174, 172) relative to the plunger arm in a case where the first gear (174, 172) and the plunger arm are engaged and the second gear (176, 178) is in the second position.