So my friend has Tiger and would like to install Leopard, and I remember reading somewhere that you could install it without losing any of your files? Is that true and how do I go about doing that? I know you can install Leopard with an external hard drive, but my external is a bit different in that it is a portable drive. It has no external power source but simply powered by the USB. Will that still work? I guess it's almost like a giant flash drive.