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If the backup in question is an iTunes library, you can use this article to select that library on the external drive and use it from there, or copy the entire iTunes library to your internal hard drive and open that copy.
iTunes: How to open an alternate iTunes Library file or create a new one
Then you would need to set the default library location to the one if it is anywhere other than the defult location.
Where are my iTunes files located?
If it is not an iTunes library then you can drag the folder into the iTunes window that has all the music in it.
Adding music and other content to iTunes
Also as a side note, you can select the folder that has all the music in it and click open, when you are in the File>Add folder to library prompt.
All the very best,
The main differences between iTunes 11 and earlier versions are the loss of coverflow and ability to have multiple windows open.
In Windows, you can restore much of the look & feel of iTunes 10.7 with these shortcuts:
- ALT to temporarily display the menu bar
- CTRL+B to show or hide the menu bar
- CTRL+S to show or hide the sidebar
- CTRL+/ to show or hide the status bar (won't hide for me on Win XP)
- Click the magnifying glass top right and untick Search Entire Library to restore the old search behaviour
- Use View > Hide <Media Kind> in the cloud or Edit > Preferences > Store and untick Show iTunes in the cloud purchases to hide the cloud items. The second method eliminates the cloud status column (and may let iTunes start up more quickly)
- If you don't like having different coloured background & text in the Album (Grid) view use Edit > Preferences > General and untick Use custom colours for open albums, movies, etc.
- With iTunes 11.0.3 and later you can enable artwork in the Songs view from View > Show View Options (CTRL+J) making it more like the old Album List view
- View > Show View Options (CTRL+J) also contains options to change the sorting of grid based views
The two entries in red will give you access to File > Add folder to library.