I have been working on my htpc for over a yr. it is constantly evolving and always changing. Originally I was running an old PC using xbmc and plex. I have since replaced that computer with a newer but still old computer. Win 8, 4 GB ram and 2.9 ghz dual core. I now only use plex. For me Plex is absolutely perfect. The main reason I replace the old computer was because it could not handle the three 3 tb hdd I purchase. There was not enough space nor connections. Nor did it recognize the 3 TB. The new computer recognizes it but splits them into 2.2 tb and .7 tb drives. Not a big deal but still annoying. I am now running out of room on the 9 tb and need to add another few hdd. So here is the question. Should I spend the money and buy a nice computer and make it the primary plex server with the other computer as a second with the less used hard drives. Or should I keep the current computer and purchase a NAS. Currently I stream to multiple places both internally on the network and externally. 24/7 it is streaming to a least one device and downloading at the same time works well. Doing anything else on the computer ( like organizing files, unzipping or even browsing) is a choir. Either way I need to expand, what do you suggest?
Good question - sounds like you're definitely getting the most out of your current HTPC setup.
First things first - you should be able to access the entire 3TB of your drives in one partition with a new motherboard - typically vendors provide an app that allows the OS to access the entire 3TB. You might check the manufacturer's website for your current motherboard to see if there's an app available for your model.
Now, secondly, it seems like you have two significant limitations with your current PC: (1) space for additional hard drives and (2) performance while multi-tasking. The space issue could be addressed simply with a larger case, without changing any of the other components, or alternatively using a NAS, as you mentioned, or even external hard drives via USB 3.0. I wouldn't recommend a full system upgrade just due to your need for more storage.
The performance issue you mention could be one of two things. If you have an older model dual-core that lacks Hyperthreading, you may be running into the limits of your CPU. Any later-model i3 dual-core, such as the 2.9GHz i3-4130T, wouldn't have problems multi-tasking, so the answer depends on which dual-core you have.
Another concern is whether you're operating your OS off of one of your mechanical storage drives. If they really are streaming/recording 24/7, I'd make sure to get the OS onto a solid-state drive. That would be a significant boost to overall multi-tasking performance.
To summarize: if you have less than a Core i3, you should probably upgrade the CPU, if you already have an i3 but not an SSD, add an SSD for the OS. And if you have both an i3 and an SSD, it sounds like all you really need is a NAS for additional storage.