An auxiliary drive mounted to a reel mower cutting unit includes an auxiliary drive input shaft engageable to a cutting reel shaft of the reel mower cutting unit. The auxiliary drive transmits rotational power to an attachment rotatably mounted to the reel mower cutting unit. The auxiliary drive has a housing with a flange positionable on a boss concentric with the cutting reel shaft. A plurality of holes extend through the flange, and a circumferential groove extends at least partially around the boss. A retaining clip is inserted through the holes in the flange and into the circumferential groove to axially retain the auxiliary drive housing to the boss while allowing the auxiliary drive housing to pivot on the cutting reel shaft.