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 and a second gear operatively engaged with each other, the first gear engageable with the plunger arm ; the second gear supported on a shaft having a longitudinal dimension, characterised in that the second gear 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 to allow the first gear to engage the plunger arm ; and the second gear configured to be moveable to the first position to reduce the range of rotational motion of the first gear relative to the plunger arm in a case where the first gear and the plunger arm are engaged and the second gear 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 and a second gear operatively engaged with each other, the first gear engageable with the plunger arm ; the second gear supported on a shaft having a longitudinal dimension, characterised in that the second gear 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 to allow the first gear to engage the plunger arm ; and the second gear configured to be moveable to the first position to reduce the range of rotational motion of the first gear relative to the plunger arm in a case where the first gear and the plunger arm are engaged and the second gear is in the second position . <EOS>