I have the HAF 932 Advanced with my modem (4 inch square) sitting on top of the case above the fan. My modem tends to overheat so I figure the air movement will help keep it cool. But will putting the modem over the fan create too much resistance for the case fan, making it work harder and potentially damaging it?
While it is true that blocking the air flow from a fan will cause it to have a greater load and draw more current which will cause it to get hotter this shouldn't cause the failure of the fan although it may shorten its life as the life of a fan is largely determined by its running temperature that it is subjected to. You must have some air flow over the modem otherwise the fan will be doing nothing and providing that you have some air flow I would not worry about the fans life expectancy.