I just set it to something custom. I actually took the green icon and turned the hue so the drive is red instead of green. It seems to hold its custom icon.
You can still do that if you prefer green. If you ever see it green, or can get it from another drive elsewhere, just copy it and paste it. Even if it's using the default, get info, copy, and immediately paste. It'll set that as its custom icon and the system will stop trying to (and occasionally failing to) override it, and will always display the custom icon you've set.
