Well, I've been contemplating this for years but I've only just now come across DrivePool, which is exactly what I've been hoping for! I got a little excited when Windows 8 was released and Storage Spaces looked like what I was after, however I discovered its (unacceptable) flaws after some research, then stumbled upon DrivePool!
First, my current system started as a large gaming/CAD workstation and has since ballooned into an always on media server too. Now, I'm really struggling for space and SATA ports (I have installed PCI to SATA adapters, PCI to USB and then to USB to SATA adapters,