Just curious....but is the 5v USB fan mod on these gridseeds that have the failed USB ferrite bead?  It could be the added current usage of the fan stresses it enough over time to the point of failure as I'm sure the USB circuitry wasn't designed with the load of the fan in mind.  I'm not saying it can't handle it, because there is plenty of power from the USB port for a fan, but given the build quality of these things it really wouldn't surprise me.