Original iTunes Library Gone, How to ReBuild Library On New Drive?
The simplest way (fewest steps) is 


Copy the iTunes folder from old (disk 1) to new (disk 2)
Start iTunes with the option key depressed and select the library from the new (disk 2)


Do edit your post or comment back if there are additional factors to take into account or if this doesn't work for some reason. Assuming your backup is good, complete and you preserved the correct location of all the files - iTunes will remember the new disk as long as it's always connected before it starts. 

If things look good - I would check the following as well


Verify the iTunes Media Folder Location in "Advanced Preferences tab" is pointing to the correct folder on new (disk 2)
Select File -&gt; Library -&gt; Organize Library from the iTunes menu to have iTunes apply the current settings from the previous step.


Down the road once you are sure things are working - it might be better to keep only the Media Folder on the new (disk 2) and move all the other files to your home folder. This lets you start iTunes for iOS device syncs and such when the external is not connected. I wouldn't do both things the same week. Just get things working for now :-)