The most important thing in video server is performance along the HD->OS->network protocol->LAN path. An OS processing video data from the HD and feeding it to the LAN must deal with the high overhead and complexity of the network protocols in question. Microsoft network and TCP/IP protocols are not designed for transport video of any type including compressed video (uncompressed video will crash the protocols). They transport as if they are just regular files, but video files are not regular files! Now add other layers of network services like DLNA, JRiver and things can get pretty stressful, especially when more than one video streams are being processed, or high bitrate (>15 Mbps) HD video are involved. Moreover, if you want to serve BD quality video (25-35 Mbps) and to more than one destinations, you need to build powerful hardware. Commercial video servers have the most powerful CPUs and multiple gigabit LAN ports precisely for this reason.
A HD being read for HD video during serving will have trouble being written to, and likely will cause video problem. Try not to do both at the same time. I.e. separate serving HD from those used for editing or receiving video content. Try to avoid using video HDs as RAID. RAID imposes significant overhead and frequently most go through a RAID 'housekeeping' cycle to ensure data reliability. When you stress a RAID for video, which require constant high-speed reads for long period of time, the 'housekeeping' cycles of RAID can cause the video stream to be disrupted. RAID 5 is for reliability, not speed. RAID stripping can be used for speed but require careful design.
Make sure you LAN is 100% gigabit qualified including the Ethernet hub. Try not to cross a router with video - consumer routers can't handle it. Wireless video? OK if the bitrate is very low, but wireless is notoriously bad for video due to extra protocol overhead which imposes severe timing restrictions to video. You must know the type of video (especially maximum bitrate) you intend to build the video server for. Then add max number of concurrent streams, additional app layer overhead (DLNA, etc). Add up total required path bandwidth, multiple by two. Then find a set of hardware (motherboard, CPU, HD, etc) that can deliver that bandwidth inbound and outbound.
Bear in mind, the most demanding, the most complex, the most prone to failure of all commercial server designs is, yes, the video server. You are not building a commercial server, but do keep this in mind. Very few in the world is able to do a YouTube video server infrastructure.