well i don't have the same drive, but i have a WD 120GB SE IDE on the internal IDE chain and have no issues with it what so ever, what it sounds like is the external drive don't support the deep sleep function of the Mac or OS X, when a piece of hardware doesn't support the deep sleep function of the Mac then what you explained happened is what the result is.

one easy was to fix it is disable the computer goes to sleep after a set period of time. i don't know if HDD sleep is part of the issue but test with it enabled if you still have issues disable that as well, the only power saving feature that wont affect the drives is the monitor sleep.

i dont know what else to tell you, maybe get another drive