I have a 1TB drive from OWC that is both Firewire and USB (Bus Powered)
http://eshop.macsales.com/shop/firewire/on-the-go
It is partitioned into 5 partitions
10.5 Diag - 30GB
10.6 Diag - 65GB
10.5 Installer - 13GB
10.6 Installer - 13GB
and Extra - that holds files. - 810GB
On the 10.5 and 10.6 Diags - the tools I use the most are:
Disk Warrior
Disk Utility
Data Rescue 3
Drive Genius 3
WhatSize
Also installed are the following:
SuperDuper - Carbon Copy Cloner
EntourAid
EasyFind
PhotoRescue
SpeedTools
TechTools
On the Extra Partion - I have all sorts of updates and program installers
IE and Apple Folder with Installers and Updates for the most common Apple items (OS updates, iLife Disc Images, Etc...) Adobe Folder with Installers and updates, Microsoft Folder with Installers and Updates (Office 2004-2011), Retrospect folder with installers, Parallels folder with installers - Updaters - and Windows Disk Images, 3rd Party Supplemental Folder (Flip4Mac, Perian, Onyx, Dock Library with Revamped skin, Dropbox, Evernote, Firefox, Chrome installers) and a Misc DMG's with everything from 1Password to Toast Titanium installers
I am actually going to get another of the 1TB drives, so I have a secondary "Diag Drive" .........
I have gone one step further, and bought 2 USB flash drives ... formatted GUID and then Super Duper copied 10.5 and 10.6 digs to them ( 10.5 is a 16 GB flash, and 10.6 is 32GB flash)
This gives me a total of 5 Diags that can be in use at any given time. Plus installers and updates for most software.
I hope this helps.