Booting up from external hard drive to upgrade

I want to upgrade mac os x leopard to lion and i have the files on my external hard drive with a ton of music and video. Can i still safely boot from this and do a clean install?

Not sure I'm following you here.... What version of OS X is on the external hard drive assuming it's bootable?

os x lion is on the external hard drive, i have os x leopard on my mac

The best way to install Lion clean is from DVD or a USB flash drive. Also note that in order to purchase and download Lion from the Mac App store you need to be running Snow Leopard 10.6.6.

If you have already downloaded Lion from the Mac App store and have it on a hard drive, it should work without harming any files that are already on the hard drive.

yea i have the lion files in a seperate folder on my hard drive with other files, i guess my concern is just booting and installing from it messing up the other files