Full disclosure: I am the developer
Check out: http://stablebit.com/DrivePool
It's a software disk pooling solution that combines any number of disks of any size into one big virtual pool. You can designate certain folders to be duplicated on the pool. Any files placed in duplicated folders will be stored on 2 disks at the same time.
The implementation is a hard core NT kernel driver with a virtual disk. There is a full NT kernel storage stack, no user mode hacks here.
Unlike RAID and similar solutions, all your pooled files are stored as standard NTFS files on each individual disk in the pool. This means that you can simply plug in any pooled disk to any system that can read NTFS to get at your files in case disaster strikes.
It's commercial software, $20 USD per server.