Sounds like an ambitious plan to me.
With most usb hard drives currently sporting USB3.0 interfaces, this may be feasible.
Of course, 3.5" drives usually require their own separate power supply via a mains power adaptor. This is usually 12V so you would need to construct a reliable 12V regulated supply that will run from your emergency power supply. (it will be more efficient to use a battery powered regulator than to use an inverter with the existing power adaptors.)
You might want to look into the
WD Personal Cloud solutions as a starting point. Their Linux compatibility situation is a little unclear, however they run on a Linux kernel, so it may be possible to tweak/hack their OS for better Linux compatibility?