USB - been there
USB static interference is a pain in the butt for Apple updates - have had several probelms with it in the past. Best fix I've found is before doing the update remove all USB devices excepet the iPhone, power down the machine, unplug it, hold the standby button down to drain all power from the machine totally (well, as much as poss) and then have a crack, preferably from a port furthest away from the power supply.
Took me three hours to update to OS4. No way I'm bothering for this one.


