Upgrading iPod Classic HDD Can I put a 160GB (late 2009) iPod's HDD in my 6th gen 120GB iPod?  Will this work, or are there other things that I would need to modify to get more storage?
Yes, you can and it'll work.
Detailed guide for replacement here: http://www.ifixit.com/Guide/Installing+iPod+Classic+Hard+Drive/564/1