No, it definitely isnīt the Hardrive sleep. Today LimeWire crashed while using it and my hard drive definitely wasnīt asleep.
I also did some tests where I let my harddrive spin down and switched to limeWire then. It worked perfectly and my HardDrive spun up again.
Although I canīt confirm that when LimeWire is in the backround, the Drive spinns down and LimeWire trys to read from it.. it doesnīt lead to a crash.
cu Martin
p.s.: (Sorry for the bad english)