I have a G5, and in the Sidebar snapshot below (which I have edited with color regions), you can see that I have two Hard drives mounted inside the machine (no added color rectangle). I also have several other drives that are hard drives mounted in "Enclosures" that have USB 2.0 connections to the computer. Where they have similar names plus a numeral, these are actually the same physical drive divided into separate volumes*. (These are designated in the green rectangle)
I connected old G4 to my G5 to use the Hard Drive in the G4 for video capture and rendering.
It turns out that USB 2.0 External Hard drives are NOT as fast as Firewire, in fact can be MUCH slower, despite what you are told by the salesman. Connecting two computers by Firewire in Target Disk Mode allows you to use the other computer's main drive as if it were a Firewire external hard drive.
I had long ago reduced my no-longer used G4 to a single, small-drive system. But Saturday, I opened it up and added the large drive with 5 volumes (partitions) on it.
But first, I had to make sure the following was true:
1. The drive I want to use from the remote computer HAS to be in the Master position on the cable/ribbon. Ribbons have three different colored flat connectors: Blue, which mounts to the motherboard bus, and two others which are at the other end for mounting two drives. Gray is the second or Slave drive, and Black is the Master.
2. You can only use the "Ultra ATA bus" -- meaning the main bus if there are more than one. Don't worry if you don't understand "bus."
In my case, [with the power off] I moved the main hard drive in my old computer to the slave (second, gray) connector and added my new drive to the master position (first, end, black) connector, connected the power connectors to each (the little milky-white plug with red, yellow, black and white wires), mounted the drive-holding cage back into the computer, and closed it up.
3. You must not have ANY other Firewire devices (camcorders, DVD drives, etc.) connected to the Mac you want to connect your Target Mac, to. My older G4 is my Target Mac, powered down, but my G5 is my daily-use Mac, and I disconnected all Firewire devices from it. You can connect and disconnect Firewire items "hot" -- you don't need to power down first.
4. On my G5, I opened a Finder window so I could see when the Target Mac showed up on it.
5. The process for starting up in Target mode is simply to press the power button on the Target (my G4) and hold down the T key alone until the main drive shows up on the other computer. That's all there is to it. On the G4 Mac, I simply powered it on, holding the "t" down on the keyboard of the G4 and waited until the drive showed up on my G5.
(When you want to turn it off, simply "Eject" the target drive from your Main computer (G5) and power down the Target computer (G4).)
This works with just about any two Macs that can be connected with Firewire (make sure they have the correct connector ports on each computer.) Another bonus is that the hard drive in the Master position doesn't even have System software on it. I cannot start up the G5 from that drive, yet it still mounts on my G5 in Target Disk mode.
Basically, this was like having a very expensive Firewire Enclosure which looked exactly like a Dual 1.0Ghz, Mirrored Doors G4. But it did cut my rendering time down to about 1/3 of what it would have been on a USB external.
* I partition large drives into 50GM or more for video editing. Each partition can be "Erased" using Disk Utility which is a fast way to return each to "Pristine Condition" -- even invisible stuff is deleted, which is good for video editing, since video software usually utilizes only 'contiguous free space' and ignores fragmented free space.
