My MacBook (Retina, Mid 2012) currently has issues with its trackpad based on water damage earlier in the year - from time to time, the mouse pointer flicks around uncontrollably (although at other times, it's fine). I know I can probably get it fixed, that's another long story outside the scope of this question - assume it's unfixable for now.
Currently I have 10.9 (Mavericks) installed. I use an external USB mouse on a day-to-day basis, with the "Ignore built-in trackpad when mouse or wireless trackpad is present" option ticked, which prevents the flicking mouse pointer behaviour.
I am considering upgrading to 10.10 (Yosemite), but I've been putting it off for sometime, because I'm concerned there may be a phase during the install where USB devices are disabled and/or my external mouse might be unavailable. As well as being unable to use the mouse, this might unintentionally mean that I "click" the mouse pointer and press something with a serious impact.
Are my fears founded? Are there phases of the upgrade where I can't use a USB mouse?