The easiest method is to drag the library to an external HD that is formatted OS X Extended (journaled) with ownership set to be ignored.
Next would be to do the following and then use a backup application that can do incremental backups, i.e. copying only those new or modified files. Two that come to mind are Synk Pro and ChronoSync.
A possibly better solution is to use the EHD with Time Machine to backjup your boot drive with the library on it. That way every hour there's an incremental backup of all files that have been added or modified in the last hour. You'll always have a current backup. However, when you're away from the EHD the backups will cease if you turn off Time Machine or be made to your boot drive which can cause it to fill up fast. I'd turn it off when you're away from the EHD.
OT