About the storage for fog server in Ubuntu
If your old computer is a server with a physical raid card, that is your easiest solution.
If you don’t have a hardware raid card then it gets a little more complicated. You can either setup a software raid using mdadm in ubuntu. If you don’t care about data loss you can setup a raid-0 configuration to stripe all of the drives. If you do care about data loss then use raid-1 (2 drives) or raid-5 (3 drives with 2 data drives and one parity drive). then mount that raid volume to /images (before fog is installed)
You can also use LVM and add the 3 drives to the LVM pool where the root directory is mounted. Again this approach doesn’t protect against data loss of a single disk.
There are a number of ways to go about it some easier than others.