Problems With USB HD When MB Awakes

Anytime I "wake up" my MacBook and try to access my USB HD through Finder or iTunes the little rainbow appears and my machine freezes on me. It leaves me no options but to force restart manually. It's happened 3 times in the past 24 hours. I really would hate loosing date on my USB drive because of this or mess up my OS with the forced restarts.
Anybody knows what's going on? Does my USB HD go to sleep when left unused?
Thanks.

Anybody? It happened again. This time my MacBook did not go to sleep but I didn't use the USB HD for a couple of hours. I tried to play a song in iTunes and the little rainbow appeared. It had me waiting for 5 minutes until I realized it would not work. So I had to force restart the machine...