I have an idea to build a multi-node Raspberry Pi PC running some distro (not Raspbian). But, the problem, I need to know how to hook up the Raspberry Pi's to work as one and it'd be nice to hook it up to some central hard drive, (if possible). And, is there a way I could do all of this manually?
It really depends on the type of work you're wanting to do. Can you be more specific?
You could create a cluster, with a master distributing jobs to the slave nodes -- docker swarm is a possibility. I'd suggest Hypriot for your distro in this case.
Another option would be to have a load balancer, such as haproxy, listening for requests which would then be distributed out among the slaves.
In terms of sharing a drive, you can't do that, but you can have a shared drive via either NFS or weedfs would be good choices.