Data in "Contagion" (S2E11) trying to get his totally unique, irreplaceable arm sliced off by putting it through an alien portal that he only just discovered.

It sure seems that, despite his being a nearly entirely unique being of special technological wonder and an overall boon to both Starfleet and sentient life in general, Commander Data sure seems pretty blasé about putting himself in mortal danger, to the point that Ben and Adam (and many others before and since) have remarked about his possible suicidal tendencies, with jokes often made in regards to missions or activities that could result in death being perfect for Data and his insatiable urge to end his androidy existence.