Well, I found that out when trying to install on my machine. Even on my older ASUS K8VSE DLX it did it. When booting from the DVD and it's loading the drivers for the drive I usually hear the drive spin up, which I guess that it puts it to sleep then wakes it back up.
If all doesn't go well and the install fails, the HDD is not recognized until pwr off, then reboot.

Just from my experiences