If you decide to use the target disk mode route, here's a warning: sometimes, OS X will mount an external drive in a way that ignores UNIX permissions. This enables any desktop user to access file on the drive, which is nice, but it will break a LOT of things if you try and install packages onto the external drive.
The solution is to unmount the drive and remount it manually from the Terminal. Once remounted, it will no longer ignore permissions, enabling you to correctly install packages onto the drive (thus fixing interrupted installs that broke the system, like mine) or do other things, like repair permissions. Open a Terminal window and run mount. Look for the FireWire drive in the list, note its /dev/ device, and then unmount and remount it -- you'll need to be an admin to do this.
$ mount
...
/dev/disk1s9 on /Volumes/Foo (local, journaled, nodev, nosuid)
...
$ sudo umount /dev/disk1s9
Password:
$ sudo mount -t hfs /dev/disk1s9 /Volumes/Foo
$
You should now be able to use the drive as usual, with the permissions intact.

