Firewire 400 Transfer Speeds Slow

Question

I have three Western Digital My Book Studio drives hooked up via Firewire 400. As I understand it, FW400's theoretical max transfer rate is 50MB/s (400Mb/8~=50MB). However, the transfer between the drives moves along at a paltry 12MB/s.
If I hook up a single drive, or all three (daisy-chained), and transfer directly to my internal drive, the transfer speed sits at 14MB/s (internal drive is a 320GB 7200rpm Seagate drive).
When I hook up one of the My Book Studio drives via USB, the transfer speed hovers around 34.5MB/s.
When I hook up a WD Passport Elite via USB, the transfer speed stays at a constant 30MB/s.
I have updated the firmware on all of the drives, and even installed WD's special drivers. None of this has helped the speed issue.
Please help me fix the slow transfer speeds!

Answer 1

That test isn't likely valid, because OS X should have cached that file in memory (unless you let the file get bigger than it has RAM, in which case it may still have a good portion of it cache). Look at the Activity Monitor and you'll see that the write speed will spike way above the read speed (if it writing drive is faster than the reading one).
_Write Directly to Internal Drive: 65.95MB/s_
cat /dev/zero | dd bs=2m of=~/big_file
851443712 bytes transferred in 12.311725 secs (69157141 bytes/sec)
_Write Directly to Firewire Drive: *25.9 MB/s*_
cat /dev/zero | dd bs=2m of=/Volumes/Khazad-dum/big_file
297795584 bytes transferred in 10.953078 secs (27188301 bytes/sec)
_Read Directly from Firewire Drive: *12.58 MB/s*_
cat /Volumes/Khazad-dum/anotherbigfile | dd bs=2m of=~/anotherbigfile
317734912 bytes transferred in 24.071077 secs (13199863 bytes/sec)
_Without bs=2m: *13.5 MB/s*_
168837120 bytes transferred in 11.914134 secs (14171162 bytes/sec)