On a new Antec case with power supply fan and a 120MM rear fan that exhaust's, I am adding another 120MM Antec Smart Cool to the front fan location.
The instructions suggest for best results use as a case exhaust fan. That makes sense for the rear.

What would be best for the front case fan in that there will be 6 HD's directly behind the front panel? To blow cool outside air over the Hard Drives or exhaust the heat from the HD's out of the case.

My goal is to preserve the life of the HD's.

So the scenario's are:
1. Exhaust out the rear with intake in the front over the hard drives or;
2. Exhaust out the front and rear and let the vents bring in cool air.

D.D.

Both my cases are drawing air in through the front fan.
Has worked just fine for me over the years.

Typically, intake in front and exhaust in back. Should apply to you. Since most if not all PSU blows air out.

Just don't put the hard disk(s) to close to the intake fan.