I have spent dozens of hours over the last years trying to find out why I get the error message "...not authorized to copy" when I try and drag a download from ADE to my nook. Why is this simple procedure not foolproof. There are hundreds of people sear

Question

Typical - what on earth am I doing at this page and why was I sent here
Now I get a message saying there is an error and my content could not be saved
What is wrong with you people?
How can you make such a simple operation so complicated that it is festooned with error messages
LEARN THE MEANING OF FOOLPROOF YOU IMBICILES

Answer 1

Unfortunately you've discovered too late how important it is to maintain an up-to-date backup of your iTunes library (and all other data of value).  You could, before wiping the drive, have considered making use of a commercial data recovery service that could (albeit at considerable cost) have extracted your library from the hard disk, even if virus infected.
In the absence of that option, you will need to restore the content of your library from its original sources:
Depending on your location, you may be able to re-download any iTunes Store purchases that are still available on the Store
Likewise, most digital purchases from Amazon (including auto-rip copies of purchased CDs) should be available from the Amazon Cloud and via the Amazon Music application - the same may be true of other commercial sources for digital downloads
Content imported from your CDs will have to imported again
The specific situation that you describe regarding the music imported from your friend's external HDD suggests that either the source is badly organized and/or originates from a source other than iTunes (other media players may use alternative tags for information like artist, title, album, etc. that are not wholly consistent with how iTunes handle these).  Without details of the issues you're seeing it is difficult to suggest a remedy other than going through the media album-by-album, track-by-track, and correcting the inconsistencies.
In the absence of a backup or access to the original library data there is no option other than painstakingly recreating your library as described above.  As you do so, you'll now realize how important creating and maintaining backups are - in my case I have at all times three separate duplicates of my library, in two different locations, where none is ever more than a week old compared to the content of my master library.