What do you miss out on if Legacy USB is diabled? Nothing really, except if for some reason you boot to DOS you can't use the keyboard or mouse. You can still hit the hotkeys to get into the bios, but you cannot hit F8 to get into the Windows XP boot menu.
All your USB stuff still works once booted.
Yay for you.
On a related note, the Western Digital backup software that comes with the drive would be awesome... if you let you backup anything other than your main drive. I want to mirror two drives. Won't let you do that, just your main drive. Lame.
Uninstall...
Update: Reinstall, and firmware update. After some issues with "Delayed write failed" errors I decided to reinstall the "smart" software and just turn off all the features. Having it there in the background seems to make the drivers happier.
Also, my iRiver e100 MP3 player refuses to work ("device is unrecognised") when the My Book drives are enabled. Unplugging them fixes the problem.
I didn't have any of these problems with my older My Book, only the newer one I just bought.
Oh well. Workaround process documented, problems solved.