When you use the term "library" with iTunes it means everything in the iTunes folder, therefore you cannot have your library on the iPod, only a copy of all media.
About iTunes library files - https://support.apple.com/HT201610 - Does not mention that a complete working library also includes other files and folders also in the iTunes folder.
More on iTunes library files and what they do - http://en.wikipedia.org/wiki/ITunes#Media_management
What are all those iTunes files? - http://www.macworld.com/article/139974/2009/04/itunes_files.html
Where are my iTunes files located? - http://support.apple.com/kb/ht1391
The central database file for iTunes (iTunes Library.itl) contains the location of each file you have added to iTunes. If you copy or move files to a different drive these locations become invalid. Now comes a critical distinction. Did you have your entire library on the Seagate or did you have only media?
If you had the entire library/iTunes folder:
- Start iTunes while holding down the option key. When prompted select the iTunes Library.itl file in the iTunes folder on the external drive.
If you had only media on the external there are several independent things to try:
- If you copied to the new drive using exactly the same folder structure as the old drive, you can trick iTunes into thinking nothing has changed by using the identical name of the old drive for the new drive.
- If you use iTunes' default organization structure try to play a broken link ( ! ) then find the file on the new drive iTunes should play. When prompted tell iTunes to try to find other files (may take some time).