The Op has only a rear fan and with the side off it will draw air from the side across the mother board but it will not cool the hard drives which are normally mounted in the front of the case
What?
If the side panel is off, the rear fan cannot draw significant air across the mobo: it will draw air from its immediate environment. Case designs dictate a closed case which draws air from the open end of a closed tube (a grill in the front of the case) and then expels it through the rear fan port. True, if there are no HDD bay fans, then the HDD temps may increase depending on the ambient temp of the room (v. the closed case) With the side-panel off, HDD fans, if present, will still push or pull air across the HDD bay. Removal of the panel might augment the action of the HDD fan by allowing more air to be moved in the event the fan creates a positive pressure within the case. In fact, removing the side-panel at the worst, will not change the present state; in addition to the above, it may also it may improve cooling by reducing the ambient case temp.
It is at this is the point where many stop thinking. They know the case is "supposed" to be closed in order for air to be channeled through the case. However, with the side panel off and a high volume, high velocity fan blowing air directly on the case components, no channeling is needed; the fan is going to push massive amounts of air in a 360 degree radius cooling your case, all of its components, and your cat sleeping in the next room.
Modest-cost cases (let's call them non-power-user or non-gaming cases), ordinarily have a HDD bay fan and an exhaust fan. Air enters via the HDD bay fan, is warmed by the HDDs, is further warmed by the gfx card if present, is warmed by the mobo, is warmed by the CPU, is warmed by the PSU and exists the rear of the case - at no greater velocity than it entered. As a result, ambient air temps in the case increase - making the already warm "cooling" air even warmer.
Remove the side panel, which automatically decreases the case ambient temp, increase the volume/velocity of air 5-10X using an external fan: now that's cool.
