Now this is weird! (Wired and weird!) I temporarily connected one of my 10 watt LEDs, mounted on its heatsink, to the 10 watt driver for it. Plugged in the power, and it came on. Made an appropriately bright circle of light on the floor, with a well defined boundary, as it should be. So, I lugged everything to my dining area where I set it up to measure PAR and take some photos. Plugged it in and nothing happened. So, I reversed the plug in the wall socket - nothing happened. So, I used an extension cord, with a neon light in the plug telling me the power is there, plugged the LED driver into the extension cord - nothing happened. So, I got my multimeter, and measured the DC voltage at the output of the driver - got a 0 to fluctuating reading. Repeated it a few times - mostly got zero. Unplugged the driver. Just for fun, measured the output voltage for the presumably dead driver - got a few volts, which dropped to zero. What the heck is going on??
I checked the polarity of my connections to the LED several times, and it was correct, for sure. I'm thinking of repeating this with the other driver, but I wish I knew what was going on before I try that.