Disks are only reformatted if you add them to the WHS Storage pool. If you just plug it in, it will initially show up as a separate drive just like on any other machine.
Velocity Micro Cinemagix Grand Theater, C2Q, 2GB, 2 TB, 3 DCT, 2 HDHomeRun
Homebuilt C2Q, 2 GB, 1.5 TB, HVR-1600, FireSTB
Dell E1705, T7600, 2GB, 250GB, HVR-950
WHS, WRT600N, DMA-2100, DMA-2200, XBox360