Larry Drake -- who played Benny on "L.A. Law" -- suffered from a unique disease that caused his blood to thicken ... and ultimately contributed to his death. According to Drake's death certificate -- he had been diagnosed with polycythemia vera -- a rare form of cancer related to an…

Emmy winning actor Larry Drake, who famously played Benny on "L.A. Law" died Thursday ... TMZ has learned. We're told Larry had some health problems lately, related to his weight, but nothing acute or terminal ... as far as his rep knew. Sources on scene tell us Larry was found in his Hollywood…