Recently, I had the opportunity to diagnose a problem with an external Firewire drive and Mac OS X. The drive would no longer mount on the OS X desktop and the only place you could see it (other than Terminal) was in Disk Utility. If you ran Disk Utility – Verify or Repair, the message: invalid content in journal would appear. The fix to make it mountable again was to do the following in Terminal.
The first command gives you a list of all drives and in the right most column, you will need to identify what the Identifier Name is for your volume. Once you have that, run the second command and substitue the IDENTIFIER_NAME with the correct Identifier Name for your volume.
/System/Library/Filesystems/hfs.fs/hfs.util -N /dev/IDENTIFIER_NAME
If it successful, this will mark the disk as no longer Journal, so you can now shutdown the Firewire drive and then unplug it. Wait a minute or so and then start it back up and plug it back into the Macintosh and then you should be able to see the drive mount again.
This allowed me to mount the drive again, but I believe there is still a problem with this drive, so my recommendation was to backyp the drive right away and then reformat it and exchange it for a new drive. I am not sure if the problems with the 1.5TB Seagate drives in RAID configuration apply to other Seagate drives as this one was a 1TB drive, but you never know.