is there a tutorial around for installing Armory on an external hard drive?
It's a hot drive that will be unplugged 99% of the time.
Tips?
IMHO an external drive will give you only a small advantage in terms of security. If you have a trojan on your computer, it will not protect you against losing all of your bitcoins in your wallet.
The only way to handle bitcoins safely - even if your computer is infected with malware - is to sign the bitcoin transactions on an offline system. So better invest in a cheap old notebook, unplug it from the internet, put a fresh OS on it and install Armory for offline signing.
Or you could buy a trezor which will do basically the same without the hassle.