|Lot still to learn from some of your statements.|
"one of them is really just a backup for the AD. they are replicating to each other but the second one really isn't being used for anything else"
Both server should be global catalog holders and dns servers. The 2nd is not a backup server but is for AD failover as well as you can bring it up as the only DC if the first DC dies by seizing the fsmo roles and engaging the dhcp server. Every DC participates in user and share access authenication. That 2nd dc is busier than you think.
"I like the idea of having a raid 5, two hard drives mirrored.. maybe a total of 4 drives.. 2 smaller ones for the os and 2 large ones for everything else. all on raid so that if one goes down, the other one takes over"
Might want to get a handle on your raid levels. Raid5 uses a minimum of three drives. Two drives mirrored is Raid1. Best raid to use is Raid 10 which is a stripe of mirrored drives which is often confused by internet "experts" with Raid 0+1 which is a mirror of stripped drives.
This means you can have two drives fail in a raid10 [one in each mirror set] and still be functional whereas with a single drive failure in Raid 0+1 you are still functional. Both are one drive failure away from unrecoverable at that point.
Always always plan your raid arrays with hot spare failover drives. They make the difference of coming in on a Sat to replace a drive vs company downtime and posssible data loss.
Server hardware is built to last. I have a Tyan system I built 8 years ago that is still running a Netware 5 server. This is not a area to skimp on.
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