syncing ipod/itunes with external HD
i know similar questions have been asked on here, but i'm having a hard time understanding.
i have a pretty large music library (~13,000 songs, 100 gigs) which obviously takes up a great deal of space on my macbook. i have all of it backed up onto my 1TB external hard drive. i want to clear up space on my mac but still be able to add music to my ipod on a regular basis (and keep all the existing songs on there). how would i go about syncing my ipod classic with the library on the external hard drive so i can delete most of the songs off of my computer?
thanks in advance
MacBook Pro, Mac OS X (10.6.8)