I have the fix!!!!
I have had the same issue ever since Lion. Its been very painful not having album art preview in finder or the play button when rollover the file for so many versions of OSX. Today I worked hard in trying to solve the problem since Applecare support hasn't helped much, all they did was tell me to bring the computer in. The other main reason I got so motivated to try and fix this is because the DJ app for Mac only reads album art from the finder even if iTunes can display the album art. The other reason that got me curious is that my friends macbook pro album art worked fine, so I cross checked with his computer until I figured out a few things that helped and ultimately fixed the issue. FINALLY!
My set up:
- I have my full iTunes library folder on my external as I have over 700GB of music.
- It includes all my database files in there, which is very important as I have many playlist settings from over 7 years of iTunes activity.
1. With every new computer I have purchased, I use to just pressed option on Mac and press iTunes to locate my folder (which I have said is located on my external drive).
2. I tried today to press option again BUT locate and selected the default library in the user/music folder of my internal drive (standard setting from Apple).
3. Miraculously, all the album art appeared in both my external drive library and also today's downloaded songs on my Macbook pro. Ofcourse with the new default location I don't have any songs or data.
4. So I tried to find a way to keep this library data on my internal drive since it can help the finder display all my files properly, yes even have the play on rollover button but as well link it back to my external drive with all my 700GB songs.
5. I then went into iTunes Preferences/Advance tab and in iTunes Media folder location redirected the location to my external drive iTunes/ iTunes Media folder. If it asks to collate data, press NO.
6. Then I quit iTunes.
7. Next I copied the 4 files to my desktop from my external drive as I want to keep the data I have made with all my playlists etc. The four files are iTunes Library Extras.itdb , iTunes Library Genius.itdb , iTunes Library.itl , iTunes Library.xml
8. I then went to my internal drive User/Music/Itunes/ and dragged these 4 files I copied from my external drive into here and making sure to REPLACE the four existing files already there.
9. Next open iTunes and you will find all your presettings appear with all your music linked to your external. What this does is keep the important library files on your internal drive (that somehow makes your finder files album art appear), BUT to read also all the music mp3 files located in the external drive (action from step 5)
Presto!! All your music files whether on internal drive or external all the album art has appeared.
Hope this helps!! Let me know if there are any other questions.
I am on 10.8.1 running retina macbook pro.
(I assume this will work with 10.8.2)