Hurt..... a lot.
That drive is going to get very hot the moment you start a defrag or run a virus scan, or even if your ambient temp is quite high (remember that all things that generate heat will generate heat plus the ambient temp, so if you are at 20C and that HDD + packing foam + no-airflow could mean that your drive is running at 45C, if your ambient hits 25C then your drive will go up by another 5C and so on).
Your best bet is to suspend it in-front of the front case fan - lots of inexpensive, safe and secure methods in the link below. The reason I know, I run virus scan on peoples HDD often and they usually sit on an antistatic bubblebag (this is not a standard bubblebag which will
create static) so the vibration doesn't drive me crazy, a laptop drive can get pretty hot, a desktop drive is painful to touch...... unless there is some modest airflow. That kind of packing foam is very good at keeping things hot, put it in direct sunlight for a while, then take it out and see how slowly it looses its heat - that is the exact opposite of what you want and over a prolonged period it can fry your HDD.viewtopic.php?f=7&t=19147