Migrating music to an external hard drive for access by Windows & Mac

Question

Hey folks
I recently migrated my collection of music to an external hard drive by:
1) changing the iTunes music folder location from the default (Music/iTunes/iTunes Music) to my external hard drive through iTunes Preferences.
2) Selecting "Consolidate library" from the Advanced drop-down menu.
Afterwards, I moved my library file to the same parent directory I put my iTunes music folder in on the external (in fact, I moved my entire iTunes folder excl the music folder which was obviously already on the hard drive). When I restarted iTunes I held down OPTION and then chose the library file on my external as the iTunes library file to be used. This was all done on a Mac.
No problems - all swell. But... I moved the library file to the external because I want to be able to use the external with multiple computers from which I can import music that will automatically be copied to the same folder (the music folder on my external) and managed accordingly. When I tried to open the library from my Windows laptop at home nothing happened. The library file I copied from my Mac had no extension, so I renamed it .itl so the Windows version of iTunes would pick it up, but selecting it as the new library did nothing.
Q1: Are iTunes library files (I assume always .itl's?) interchangeable between Mac and Windows? If not this might explain why the library didn't load up.
Q2: Is my method wrong? Is there a different procedure for getting multiple copies of iTunes on different computers and operating systems to all use the same library, saved on an external hard drive, which itself references music from the same external hard drive? Is this goal even possible?
Q3: Is it possible to have 2 or more libraries for a given installation of iTunes?
Bonus Q4: How is the album artwork saved? When I consolidated my music on the external, did this move the artwork across with it (integrated with each music file?) or was it still sitting on my Mac hard drive in the Album Artwork directory? I have since moved this folder to my external as well, when I moved the entire directory including library file across, and album artwork is visible when the library is open on my Mac, but I don't know whether its getting the images from the hard drive or the external hard drive.
Okay last one - I swear! Q5: When I couldn't get the laptop version of Itunes to open my Mac library file, I imported all my music into the default laptop library which references the music files still all located on the external. Considering the 130GB of music I'm dealing with, this took a while, but what took even longer was "Processing album artwork", which dragged on for ages. What is actually happening during this procedure - is iTunes accessing an online database to find the artwork, or is it working with the artwork already processed and saved in the album on the external?
Thanks very much in advance! Sorry for all the questions but I'd like to crack this thing once and for all.

Answer 1

The procedure for sharing a library between a Mac and Wintel PC on an external drive is quite simple. I did this myself so that I would have my music available to me when I worked in my lab.
First before you transfer anything to your external hard drive, the music library on your Mac should already be consolidated. Once that is done, transfer the iTunes folder (./Music/iTunes) to the external drive. Unless your music files are in a folder outside the iTunes folder—for instance, my music files are on a separate, larger volume on my Mac from the iTunes folder that is on the boot volume—copying your iTunes folder will put everything that iTunes needs onto the external drive.
Now you have two options for using your library on two machines. The first, which I prefer, is to leave your Mac’s iTunes library in place and use the external strictly for the Wintel PC. In this instance, you would need to add the .itl extension to the iTunes Library database file. When you attach your external to the Wintel PC at work and choose the library, your should be taken into iTunes with your music library intact.
In this instance, your external drive can also serve as a backup of your iTunes library because it is highly improbable that both your external drive and Mac Pro’s iTunes folders will go bad simultaneously. For backup purposes, you will need to place another copy of your iTunes Library database file on the external drive, without the extension, for your Mac; I have had issues with Macs recognizing .itl files. Of note, the content of your library on the external drive will be the same if you need to recover from the external drive, but as the Wintel PC will not update the database file without the .itl extension, any settings you change or playlists you add in Windows will not be transferred to the Mac unless you recover the Mac’s library from the iTunes Music Library.xml file that the PC would have kept current.
In the second instance, if you wish to run your library off of the external drive for both your Mac and Wintel PC, you would follow the same procedure described above, but you would need to have both an iTunes Library (Mac) and iTunes Library.itl (Windows) database files. Effectively, this would mean that once a change is made on either system, it will not be reflected on the other. As you are most likely to add new songs via your home computer, you will need to replace the .itl file with a copy of the Mac database file whenever you add music, playlists, etc.
Given the storage capacity of my Power Mac G5 and the logistics involved in managing a library stored solely on an external volume going between a Mac and PC, I find the second option to be more trouble than it is worth. For me, it is easier to maintain my Mac’s iTunes library then copy the iTunes folder over to the external drive overnight when I wish to make any updates (e.g., new content, playlists, etc.) available to my work computer. In the morning, I just add the .itl extension to database file before disconnecting the external drive and taking it to work.
Now that your first two questions are answered, yes you can have more than one library, but doing so is poor database management. You should have all of your content in a single database and use iTunes to separate content. iTunes already separates music, movies, podcasts, audiobooks, etc., but further division of content is up to the user. iTunes has had playlists and folders for a few versions now, so it is quite easy to organize your library’s content in as simple or complex a folder/playlist structure as you please.
How album artwork is saved depends on how you acquired your music. Music you purchase from the iTunes Store has its artwork placed into a separate artwork database built in the Album Artwork folder in your iTunes folder. I believe that the same occurs if you allow iTunes to find and import album covers. If you rip CDs or acquire music from other sources, you manually add the artwork, in which case the images are stored in the music files.
Lastly, when iTunes has to build a library it can take a while. Your library is twice as large as mine and on a slow(er) laptop drive the time to built a library will be excessive. Unless iTunes is crashing or freezing, let it runs its course until the library build is completed. Also, if you have copied your music files to your laptop’s internal hard drive, you should not leave iTunes referencing the external drive. In the worst case, you would want to copy your iTunes folder from the external to the laptop’s internal drive’s default iTunes folder location. Remove the iTunes Library file and allow iTunes on your laptop to rebuild the library from the XML file.