2 rpi connected by a Ethernet crossover cable
front computer >>> rear computer
usb switch to usb hard drive connected to both rPi
These work very well to share a usb hard drive between 2 computers.
Just unmount them before switching.
The second rpi could mount it read only so you would only have to worry about unmounting on the first one.
I would have the fist rpi mount it r/w only when copying data to it.
It is possible to make one rpi control the usb switch by replacing the switch with a transistor or relay controlled using an io port.
The control link between the 2 can be done a dozen different ways.. from trivial to complex.
netcat would be my knife of choice for sending and receiving commands :}