Three years late, but still relevant. I had the same problem and FakeRaid was simply out of the question. Use AUFS. It will join the drives under a single drive. The mfs setting will put new files on the drive with the most free space. There is also rr which is round robin and pmfs which will put files onto the drive that has the folder already and has the most free space. I personally use pmfs. My setup works like so.
# The Archive
LABEL=Archive_000 /mnt/Archive_000 ext4 defaults 0 0
LABEL=Archive_001 /mnt/Archive_001 ext4 defaults 0 0
LABEL=Archive_002 /mnt/Archive_002 ext4 defaults 0 0
LABEL=Archive_003 /mnt/Archive_003 ext4 defaults 0 0
LABEL=Archive_004 /mnt/Archive_004 ext4 defaults 0 0
LABEL=Archive_005 /mnt/Archive_005 ext4 defaults 0 0
LABEL=Archive_006 /mnt/Archive_006 ext4 defaults 0 0
LABEL=Archive_007 /mnt/Archive_007 ext4 defaults 0 0
LABEL=Parity /mnt/Parity ext4 defaults 0 0
LABEL=Q-Parity /mnt/Q-Parity ext4 defaults 0 0
I added the init.d script (Due to drive mount times being too slow to keep up with aufs mount):
mount -t aufs -o noxino -o br=$d0=rw:$d1=rw:$d2=rw:$d3=rw:$d4=rw:$d5=rw:$d6=rw:$d7=rw -o create=mfs -o sum none Archive
That gives me 10 mounts under /mnt. I like it this way because I use SnapRAID which you'll have to download and compile (there are guides for it). I use this on a Samba server, so the only thing everyone else sees is just the Archive folder. Make sure to make the directory other wise you'll get a mount error.