Sputter erosion of boron nitride (BN) plays a key life-limiting role in many Hall thrusters. We report on the development of cavity ring-down spectroscopy (CRDS) as a sensitive and accelerated diagnostic for sputter erosion. The measurement approach is based upon probing sputtered boron atoms in the region of 250 nm. We present quantitative detection of sputtered BN using CRDS with a pulsed laser source. The pulsed measurement results are compared with modeled signal levels. We update the development of a continuous-wave CRDS system using the frequency-quadrupled output of an external cavity diode laser as the light source. Characterizations of the measurement sensitivity show that with integration times of several minutes, the current performance would allow high signal-to-noise erosion measurements for expected thruster conditions. Approaches to increase sensitivity (reduce measurement time) and to integrate with vacuum chambers for thruster testing are summarized.