Do you know that you can check the temperature of the hard drive by using the setup menu? Select Menu-6-3 (diagnostics), then select Counters. Use the Page Down button on the remote to scroll down the Counters list until you get to the hard drive internal temperature. If you monitor this you can get an idea if you need to use an external fan for extra cooling. BTW, if you do add an external fan, or do anything else to reduce the temperature, it takes about a whole month for the average temperature reading to stabilize to its new value.
My 722 was in an open cabinet and the average temperature was about 130F. This was about the limit for a hard drive to run reliably. I added an external USB fan and now the average temperature is 109F (much better).