And long power cables. Unless you wanted to use a separate power supply in the HD case...in that case, I guess you'd have to turn the power on on the HD case first, then your main one. You'd either have to use an AT power supply or do some rewiring to 'fool' the PSU into thinking there's a mobo in there.
Why not just buy a really big (server) case and put everything in it? It would be more expensive, but much much easier.