Hey rockinthesixstring, I had the exact same issue, and it was driving me up a wall. I had thirty gigabytes of music, and only ten gigabytes of space on my primary hard drive (C:), so I moved my library to my second hard drive (D:). BUT ITUNES KEPT RESETTING THE MEDIA LOCATION FOLDER!!! I had to manually change it back every time I opened the program, and re-add all of my files. I noticed something strange, too. All of my settings were also being reset, like the name of my library, grid colors, things like that. I even contacted Apple Support team twice, but they couldn't figure out the issue either.
But then I noticed something else: in my default folder, where iTunes was trying to force my library to be, I had the iTunes preference and library folders (iTunes Library.itl, iTunes Music Library.xml, ect.) but they weren't being created in the iTunes folder on my new drive. Here's what I did (and what worked for me!):
I had the following setup for the folder I wanted my music to be (D:\iTunes\iTunes Media). I went back into iTunes, changed the preferences and media folder to how I wanted it again, and re-imported all of my songs.
Then, **leaving iTunes open**, I created a new folder like this (D:\Music\iTunes\iTunes Media). I only did this to differentiate one from the other, and give it a structure similar to how iTunes prefers it to be. You can name the folders whatever you want. I went back to iTunes, and changed the media folder location to the new one I has just created. Then I went to File>Library>Organize Library>Consolidate Files. This made an exact copy of my initial folder into the new folder.
I then grabbed the iTunes Library.itl file from the folder in the drive (you'll probably find it in the first folder you had on the drive you want it to be on, but it may already be in the new folder) and make sure it's copied to or already in the new folder.
Then close iTunes. This time when you open iTunes, before you click on the program, hold down the SHIFT key. You should see a box open up that has options like "Quit", "Choose Library...", and "Create Library..." Click on "Choose Library..." and locate the iTunes Library.itl file you just saw, and select it.
iTunes will re-import all of the songs again, but if you go to where your new folder on your desired drive is located, you should also see all of the iTunes preference files that weren't there before.
What I realized with this is that iTunes doesn't use the iTunes/iTunes Media/Music folder to keep track of your library. It uses these .itl and .xml files instead. When we created our new media locations on the new drives, iTunes didn't re-create these files onto the new drive. Since iTunes didn't have these files on the new drive to point to when it was restarted, it silently reverted back to the main drive, where it knows those files can be found.
Once this lengthy but worth it process is completed, test out iTunes. You should be able to open and close the program and have your iTunes library there every time without it ever resetting again.
Then go back and delete all the files you had for iTunes in the first folder you created on the new drive. Also make sure that you don't have any iTunes files or folders on your main drive that you don't want your music on. You should have just one copy of your files once this is all over.
This worked for me, and I really hope it works for you, too. For ALL of you!
Happy music listening!