[[ Just to clarify, I have already got an external drive with the latest ML on it…I'd like to boot from it as it is now ( with ML and all my programs ).
Possible? ]]
That depends...
... on whether the -version- of ML that you have on the external drive is capable of booting your Mac. If you have an earlier version of the system than the one required by your current Mac, chances are it won't be "good enough".
The easiest way to create a bootable clone is with CarbonCopyCloner or SuperDuper. CCC will even re-create the recovery partition on an external drive.
Also, be aware that there are -some- external drives out there that won't boot the Mac, even if the OS is "correct". That's because of hardware (more likely firmware) problems with the drive enclosure itself. It has nothing to do with the software.
How to boot from the external:
- Restart
- Hold down the option key and keep holding it down until the startup manager appears
- Select the external drive (use pointer or arrow keys), then hit return.