USB Power stays on in standby.
Useful for charging WiiMotes, but not useful for having the hard drive run all the time when you don't want it to.
Because WiiConnect24 causes so many problems with overheating and with some games it is best to not leave the Wii on standby anyway. Only turn on WiiConnect24 when you need to go to shop/weather/etc then turn it back off.
NOTE:
Not all USB drives can be powered from a single USB port. I tried a Maxtor 250Gb USB drive and it would constantly power cycle if only plugged into one port on the Wii. I opened it up and checked the specs of the drive inside it and the drive alone required 560ma , not counting USB-SATA circuitry (USB ports are only rated for 500ma each). If the drive you are buying uses a Y-USB cable, intended to get power from 2 USB ports, it may or may not work if only plugged into one port (some will, some won't).