mhddfs (Multi HDD [FUSE] File System)

A fuse-based file system for unifying several mount points into one.

This FUSE-based file system allows mount points (or directories) to be
combined, simulating a single big volume which can merge several hard
drives or remote file systems. It is like unionfs, but can choose the
drive with the most free space to create new files on, and can move
data transparently between drives.