I aware that if anything that affects the path changes will move the files. But when the files disappeared from my HD, they were nowhere to be found on my Mac. Otherwise I would have just copied them back.
Then that is going to remain a mystery, as I don't have the environment you have to test. Given that you aren't actually saving any changes, this would be a first in that MC is deleting a file for which it shouldn't have made changes. Maybe the PL rule is applied when you edit the filename field, and that creates an issue. But it shouldn't. I can edit the [Filename] field to any garbage I like, then press Escape rather that Enter, and nothing changes or happens. The On Disk (external) function still works, and the file is there. Mystery.
Feeling a bit confused now... help!
I was just showing off my knowledge of the internal workings of MC. You don't need to know much about the Databases inside the MC Library, and you won't see them usually. But sometimes it is handy to understand a bit about them.
To see the list of Database names, and maybe their contents, create an empty View in the Audio section, then in "Set rules for file display" add a rule in the "Modify Results" section by changing "No modification" to "Limit database to", then click on the right DropDown list. You will see the names of all the Databases inside MC. You can play around with that to see what is inside the various Databases. Perhaps uncheck the "Use parent scheme rules for file display" so that you will see more than just Audio files.
I thought I was 'moving' my Library to E: by backing it up there and by pointing File Location for Library Backups to E:.
Nope. It is just like any file backup process, but puts the MC Library and Settings into a ZIP file rather than backing up the files to another location. You can't open that ZIP file as a Library, until you Restore it into an existing Library.
So, I guess I don't understand the difference between moving it and backing it up to E:. That said, never changed any File Locations on PC or Mac, so they are all still pointing to C: drive on PC and /Users/ on Mac.
To move the Library to the E: drive you would have to create a new Library on the E: drive using the "Add Library" function, and then Restore one of your backups into that new Library. Then turn off the setting in MC that says always load the Default Library (in Startup Options), then load the new Library. After that MC would always load the last Library it was using, which would be the Library on E:, unless you changed that at some time.
Note also that the location of a Library isn't defined in the "File Location" Options. That is for other stuff. The location of the Library is defined when you create a new Library.
Should I be leaving it this way or updating File Locations to all point to E: on the portable drive? I hope this isn't another can of worms...
For now, you should leave it the way you have it. Once everything is working, and you understand everything I have written above, you could create a Library on the E: drive to test if it is too slow. You can always backup that Library and go back to using the Libraries on the PC and Mac drives, if it was too slow. But if you do test and go back, delete the Library on the E: drive to avoid confusion in future.
The link for Library Wiki doesn't open...
Oops. My error. You could have searched the Wiki.
Here it is:
https://wiki.jriver.com/index.php/LibraryAm I doing the right thing leaving all File Locations pointing locally on the PC & Mac rather than to E: or /Volumes/E/? I'm wondering what the effect would be if I changed them all to point to E: or /Volumes/E/ respectively.
I think, but haven't fully checked, a lot of those location settings are MC installation specific, rather than part of the Library. I'm not sure. I would be leaving them pointing locally unless there was an issue with some functionality. Most of them are just defaults for actions you can take in MC, and you don't have to use the default when you actually take those actions. Cover Art may benefit from being moved to the external drive, as I think I mentioned earlier. But I'm not sure the PL rule would be applied to looking up Cover Art. The "Conversion Cache" and "Program Files" settings are sort of in the same boat. You would have to test that. It could be a mini project.
Fortunately, my Thumbnails & Cover Art seem to be working fine for now, but I think that may have been accidental luck because my music was originally on my Mac and when I deleted the music files I did not delete any locally stored Thumbnails, Cover Art or anything else related to MC. Now I'm questioning my understanding... Had I never had my music & MC on the Mac in the first place, how would Thumbnails & CA (or any other vital data) be recognized by MC-Mac using Portable Library? I'm wondering what I should be doing to make sure I am working from the same set of images (and other data for that matter) for both machines?
The Thumbnails Database is always located on the computer where MC is installed. It is tied to the MC installation. You can't change where it is located. MC builds and updates the Thumbnails Database when it needs to. You will always have Thumbnail Databases on each of your computers.
You can only move the Cover Art location. Of course, for Audio files where the Cover Art is inside the file or next to the media file in the same folder, there is no issue. The Cover Art is on the external drive and moves with it.
For other Cover Art, such as for TV Shows (Series and Season Cover Art), TV Logos, Artist and Composer images, Home Videos and downloaded YouTube videos, etc., right now you probably do have three sets of Cover Art images, one on each computer. If that isn't an issue, leave it alone. If you need Cover Art for those media types, and you start seeing issues, ask for help again.
Moving the Cover Art location in Library settings can be disastrous, deleting files, or whole drives, if you aren't careful. Moving the location from three computers onto one external hard drive would need a specific approach. So, for now, don't if you don't need to.