Hi. So, do you mean..how to eg: make a copy of your install onto a HD, then be able to boot off the external HD? If that's yes, then:
- Format external HD using DiskUtility in the same way as for your onboard HD.
- Get something like SuperDuper (free version available) and do a copy of your Mac OS partition onto the external HD.
- Install Chameleon bootloader onto your external HD. Or I guess you could run NetBookInstaller or something on the external HD, but that might wipe out any changes you'd made to /Extra, DSDT, smbios etc files.
If you have it configured right, it should then be bootable provided of course you set BIOS to boot from the external drive.