I would like to build a home file server running Windows Server 2012, with WD Red hard drives configured in RAID 10.
What is the least number of these hard drives I need to purchase to be able to configure in RAID 10? I've researched and some sites say 4. Is this accurate?
Do I need an additional hard drive to install the OS (Windows Server 2012) on, or will I be installing it on the hard drives I have configured in RAID 10? (That's probably a stupid question, but I just want to be sure.)
RAID 10 requires at least 4 drives. Two sets of two, since you're doing mirroring (RAID 1) and striping (RAID 0). If you're doing a hardware RAID then you can install the OS on the array.
To answer your question about installing the OS on a separate non-RAID drive, the down-side is that you will not benefit from the RAID as far as your OS is concerned. If the disk fails, you'll experience downtime and have to re-install the OS from scratch or from a backup. Since this is a home server I doubt the down-time is an issue. On the other hand, in my experience you'll have a much smoother and less complicated installation if you put the OS on a separate, non-RAID drive.
RAID 10, or 1+0, is a stripe of mirrors. Therefore, you need a set of 2 drives for one mirror (RAID 1) and at least one more set (of 2) to achieve your stripe (RAID 0). So, 4 drives would be your minimum requirement.
If you're using the RAID 10 array just for storage, independent of your server, then yes, you might want another drive for the OS. But keep in mind, it won't be on the RAID array.
If you want everything to be on the RAID array, then there is no need for additional drives. The OS will be installed on the array as well.
