Yes, i think @nfoav8or explained it all and there is nothing more i can add except that it is also my favourite method. One partition of 10gigs on USB drive with osx installed and the rest is just fat32 used for everyday work, copying and transfering files, movies. Bootable usb is used to install osx on the main drive. When something needs to be repaired you can boot it and fix your main system instead of booting with dvd. IMHO, usb hdd can help really a lot. Minimal, for example 10gb partition, is not big at all and bootable and installed osx can just sit there.
Regards mate!

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