A couple times this week, the drive belt for my floor model Kenmore humidifer jumped off. I have a spare belt off a similar model. But, that spare belt jumped also. Figure it may be working too hard, the media pad is many years old, and a bit crusty.
I had a spare media pad on hand, so I decided to try and put it on. Tuns out to be easier than I thought. Four screws on top, and the mechanism lifts straight up.
Loosen one turn knob, (right side) the upper roller comes out. Pry a bit on the lower roller, and it came out also.
The old media belt weighs several pounds. The new one, several ounces. Put it all together, and give it a test. The fan runs, and the media belt does not move. Then I noticed the drive belt, near the machine. Put that back on.
Hope that fixes it for a while.