As root run:
# fdisk -l
That will give a printout of all partitions on your system recognized by linux, whether mounted or not. See if you get anything for /dev/sda. The firewire drive may simply not be recognized. If that's the case, you'll have to see what modules need to be loaded for the device to be recognized, i.e. the firewire modules.
Note: I don't think linux can currently write to hfs+ if that's the way the drive is formatted, but you should be able to read it.