I tried running rescan-scsi-bus.sh, and the firewire drive shows up at scsi2: 0 0 0.
I'm pretty newbie-ish myself, but sounds as though you have two scsi devices, in which case your external HDD may be sdb rather than sda . Basically sd = scsi (not ide) device; a = 1st device, b = 2nd device, c = 3rd device and so on; lastly the figure after that refers to the partition.

After executing rescan-scsi-bus.sh, use fdisk (!) for a list of disks and their partitions, e.g.
Code:
fdisk -l
or you might have to type
Code:
/sbin/fdisk -l
Then do the mkdir and mount as you've been trying.