Once in a blue moon, usually after I have been slinging around a lot of files, I will get an error that a file could not be copied from iTunes to whatever device I am syncing because it could not be found. Almost every time this occurs it is because iTunes has created a bad XML key in addition to the good key that it also created when the song was copied into the iTunes library. The bad XML key tends to contain the original location of the file.
So, what happens is that iTunes can play the song because it uses the good key. It will also copy the song because it has the good key. However it will also attempt to copy the song using the bad key. This prompts the error and here's how to fix it.
You need to shut down iTunes and find the "iTunes Music Library.xml" file in the /Users/yourusername/Music/iTunes/ folder. You then copy this file to the desktop and perhaps name the original file something else. Then, open this file with Text Wrangler (free) or another hard core plain text editor. Then you need to search for the bad key. The best way is just to search for the Song Name. So, if you are searching for the song, Glass Onion, just type that into the search box and the text editor will take you to that instance.
Notice that the iTunes library file is neatly ordered with each song having a unique "key" consisting of a dozen or more lines. You are interested in the line that shows the location of the file. Look at that and see if it matches the location in the iTunes Music folder. If not, then there is your problem. If it matches, then continue your search; the errant file will almost certainly be in another key and the location will likely be wherever it was when you dragged it into iTunes.
When you find that key, then you have two choices. The simplest if you do not want to edit the XML file is to simply copy the correctly referenced file out of its folder in the iTunes Music folder to the location referenced by the key. Then, close the XML fle without saving and copy the unsaved XML file back to its correct location, Fire up iTunes and you should see another copy of the song in ts Music library. Select the song and "Get Info" on it. You should see the location at the bottom of the get info box and it will be in the errant location. Back out the Get Info, delete the file from iTunes and trash it. This will automatically delete the bad key which was the objective of this whole entire Massacreee! Check your trash and it should be there. Check your library and the correctly referenced file should still be in your iTunes library.
You can also edit the XML key by simply deleting the key. While that should work, I don't like such ham-handed whacking and you could have unforeseen problems. I have never seen this, but if the address in the bad key is nonsense, then you can put a valid location in the key, copy the song file to that location, save the XML file, copy it back to the /Users/yourusername/Music/iTunes/ folder and go from there. Finally, note that have used the word "copy" advisedly here. That is different from "move." This is a nettlesome little problem. Good luck.
