Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
This old site has a pretty good list of what hard drives you can put in a first gen iPod. Unfortunately they don't seem to work very well with SSD solutions which is a real shame.
 
I also don't think they work with SD flash adapters too. That means replacing a HDD is the only option?
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.