Do you connect to it via firewire(FW)? If so, the first thing you should try is plugging an iPod or some other hardware that connects via FW into the FW port and see if it works (i.e. mounts or connects).
Next step: Click on About This Mac in the Apple drop-down menu and then click More info - then click on firewire - what information shows up in the right hand side window(s)?
If they are blank, or even if they are not blank, you may need
to reset your firewire ports - unplug any firewire devices and
power off the computer and unplug it from the wall, wait 20
minutes and plug it back in and boot up.
Plug the FW drive into the FW port and then open Startup Disk in System Preferences - if the FW drive does not show up as a startup disk choice, log out and log back in - if the FW drive still does not show up, reboot the computer.
When you reboot, you might as well go ahead and hold down the option key after the startup chime - a simple GUI should appear that shows all bootable volumes available.
If the FW Drive does not show, choose the normal startup volume and boot it up and check System Preferences/Startup Disk again.
If Disk Doctor sees the disk, it quite certainly isn't the FW port.
Well: NEVER use Disk Doctor, btw. It's a bad, bad, bad tool that can do bad, bad, bad things to your harddrives. It has a history of bugs that turned a lot of users' data to garbage. Well, less than garbage, actually.
Instead: Use Apple's Disk Utility. Does it see the drive? Are there any volumes on it to be seen? If not, you can use Disk Utility to create some.
Disk Doctor or Disk Utility "seeing" the disk does not rule
out the possibility that the FW port is not working properly.
Resetting the FW port will help rule out that possibility and narrow down the list of possible causes.
I did forget to ask if the disk had ever successfully mounted, or if it had been fomatted or even had any data on it yet - good catch, fryke!