another take would be to act above the filesystem, by rsync-ing the data across the drives with a cron job. This would not be instantaneous, but would work on ANY os and any size of hard drive, and be very flexible: if you have 4TB + 2TB + 2TB, you can replicate the first half of the 4TB on the 2TB, and the second half of it in the second 2TB.

But then you need to manage which set of file you want to store into which hard drive. 'du' is you friend, then I guess.