Shouldn't be a problem at all. I know my 2.5" drive running in a FW enclosure doesn't even barely get warm unless transferring HUGE (1-2gb a piece) files.
I kept my Western Digital external in a drawer for about 6 months. The drawer did have pretty good airflow, and the top wasn't completely solid, and allowed the heat to rise out of the drawer without any problems. The drive worked (and works) fine.