- attach external drive
- Assuming it's new/clean, use Disk Utility to partition and format it. Choose the right partition type for your Mac (Intel vs. PPC)
- Use SuperDuper to "back up" your internal to the external
- After it is complete, test by rebooting, and holding the Option key down to boot from the external
- If that works, you can then take your external apart and move the drive to the internal location
Just make sure that you format it correctly in Disk Utility. To make it easier in SuperDuper, you can name the partition something other than "Macintosh HD". I chose to name mine after the drive model number, aka "7K200"