Exactly 
in the usb world there are 2 category of devices: bus powered and self powered
when arduino is plugged in is bus powered while when you plug the power supply and move sv1 only the ftdi chip is bus powered and the load is quite small (a few ma)
there is only one case where this can be annoying: when the ftdi reports a power need of say 100ma but it needs only 10 (because the FTDI chip is the only one powered) on certain machines the OS might say "sorry i cant accomodate that because there is too much power already used on the bus"
it's a remote event that can be fixed using an active usb HUB...
massimo