The easiest and fastest solution is to use mhddfs driver on Linux operating systems. It is a fuse-based file system for unifying several mount points into one.
The mhddfs (fuse) file system allows to unite a several mount points (or directories) to the single one. So a one big filesystem is simulated and this makes it possible to combine a several hard drives or network file systems. This system is like unionfs but it can choose a drive with the most of free space, and move the data between drives transparently for the applications. While writing files they are written to a 1st hdd until the hdd has the free space (see mlimit option), then they are written on a 2nd hdd, then to 3rd etc. df will show a total statistics of all filesystems like there is a big one hdd. If an overflow arises while writing to the hdd1 then a file content already written will be transferred to a hdd containing enough of free space for a file. The transferring is processed on-the-fly, fully transparent for the application that is writing. So this behaviour simulates a big file system.