[B]Drive[/B]
Unless you are using E-SATA, using an external hard drive for your images is going to create a massive disk bottleneck.
I dont see the reason for the separation of images. There is no added security or performance gain, so the separation is pointless.
Apart from the reasons above (which should trump this), you cannot have 2 separate image directories.
Easy answer: Spend less money, buy internal drives, set them up in RAID, get better performance.
[B]Network[/B]
I would setup DNS in each network that points the hostname ‘fog’ (or what ever your server is called) to the correct IP Address on each network. i.e. Network 1: fog resolves to 192.168.1.1, Network 2: fog resolves to 192.168.2.1
Then configure your Clients and anything else fog related to use the hostname ‘fog’ instead of an IP Address.
[B]RAID[/B]
As long as the kernel image supports the RAID you will have no issues. You may (probably will) need to download the latest kernel.