That's more or less it (mhddfs), except all drives share the exact same directory structure, or at least they appear to. mhddfs automatically clones directories to other drives as needed when storing new files in the "same directory" as existing files.

It's really quite useful and exactly what a lot of us might want. Way better than JBOD. The code in mhddfs defaults to picking the first filesystem (drive) with sufficient free space (mount option configurable) when storing a new file. And if all drives are below the user's threshold, it instead chooses the filesystem with the most available space.

It's also got a fancy feature to automatically relocate a file if, while writing to it, it runs out of space on the currently chosen drive. I don't really need that feature, and will likely get rid of it if/when I re-implement things.

So from that, one can see that if each of four drives has only 100MB of free space, it still cannot store a 200MB file --> the minor constraint is that each file must fit onto a single filesystem (drive). Not really an issue, but there you have it.

Cheers