mhddfs in action
http://www.linuxjournal.com/content/many-drives-one-folder
The mhddfs program is a userspace application that creates a virtual mountpoint. Using standard drives (USB, network, RAID, whatever), it concatenates the storage into a single volume. When you write files to the virtual volume, it just saves them to the underlying drives until one of them fills up, then it moves to the next.
