TL;DR
Change your rules to;
On my MAC Rule 1 = C:\users\susie|/Volumes/B
On my other PC Rule 1 = C:\users\susie|E:
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Unfortunately, there is a lot of detailed functionality in MC that has no detailed instructions. Users just have to get in there and try stuff.
To that end, I had a go and got it working. Don't celebrate yet though, as I don't have a Mac or Linux computer any more, so I just tested between two Windows PCs, and that is a little easier.
So, some background:
The music on my HTPC resides at "G:\Music", which is also shared via Windows sharing as "\\WILMA\Music"
I planned to do a Library Backup on my HTPC and Restore it on my Workstation, so that I can play the music from my HTPC on my Workstation using a local Library.
What I did:
First I did the Library Backup on my HTPC and copied to a location visible to my Workstation.
I then turned off Auto Import in my Workstation settings.
I then created a new blank Library on my Workstation, and made sure it didn't start importing everything it found. I have that setting turned off, but wanted to be sure.
I then Restored the Library Backup from my HTPC to the new Library on my Workstation. I did not Restore settings, just the Library and Playlists.
I then added the Portable Library rule "\\WILMA\Music|G:\Music" and tried to play back some music. That didn't work.
As an interlude, when I tried to play tracks MC told me it was having trouble tagging files. I thought to myself; "Why is it trying to tag files?" I checked if Auto Import was on, and sure enough, it was. The settings was off in the new Library, and I didn't Restore settings.
I turned Auto Import off again.
I then set up a new Portable Library rule "G:\Music|\\WILMA\Music" and tried to play back some music. That worked just fine.
I checked a few Playlists, which played fine.I noted that all files were showing the "Missing File" icon, so I rebuilt thumbnails to correct that. It required three runs before MC said all thumbnails had been built. It didn't fix all thumbnails.
I had a look at the Missing Cover Art Smartlist, which showed 2223 files still didn't have Cover Art.
Okay, that Library is still a bit of a mess. But some files showed the "Missing File" icon instead of the default thumbnail, and those files played fine. That shouldn't happen. I didn't investigate that further though. Okay, I did look around a bit more. Most of the problem was that the Library was pointing to locations other than "G:\Music", so there was no Portable Library rule and no share. The only files that have a "Missing File" icon now are those that can't be accessed by MC, because there is no share available.
So, when setting up the Portable Library rules, the first part of the rule is where the Library thinks the files are, and the second part if where the files can actually be found. Therefore your two Portable Library rules;
On my MAC Rule 1 = C:\users\susie\Music\My Music|/Volumes/B/Music/My Music
On my other PC Rule 1 = C:\users\susie\Music\My Music|E:\Music\My Music
Should have worked... as long as the music can actually be found at "/Volumes/B/Music/My Music" on the Mac, and "E:\Music\My Music" on the other PC. As you aren't using files on a remote share, you shouldn't need to do any sharing, because you are pointing to "local" files.
I'm thinking that the problem could be the space in "My Music", which appears in both the Source and Destination parts of your rules. Maybe try quoting those... or maybe not. I just tried using a rule "G:\Music"|"\\WILMA\Music" (with quotes) and nothing worked. Files wouldn't play.
Maybe the functionality can't handle spaces in the path. The configuration of the rules is pretty basic, so a simple thing like that may have been missed.
Maybe the functionality was intended to be used with remote files, such as in my test, rather than having files available on the Mac and the other PC via an external drive.
Maybe there is something else going on.
You definitely need to Restore the Library Backup again, and not run Auto Import to make the files visible in MC on the Mac and other PC.
Finally, I guess as no-one else has asked for help, maybe the functionality was thrown together quickly then launched, so see if anyone tried it. As far as I know, you are the first to ask for help with it. If people aren't using it, it isn't going to get much attention. I'm not sure the functionality was refined to even handle thumbnails properly, given my observations. It works, sort of, but not entirely.
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Wait. I just noticed the files in "G:\Music Ripped Separately", which does have a Windows share setup on the HTPC, also play correctly. But there is no rule for that directory specifically. So MC is actually just doing a simple character search and replace of the [Filename] field.
i.e. My rule "G:\Music|\\WILMA\Music" is simply taking the path;
G:\Music Ripped Separately\Yello\Stella\11 - Angel No.flac
and substituting "\\WILMA\Music" for "G:\Music" to give the new path;
\\WILMA\Music Ripped Separately\Yello\Stella\11 - Angel No.flac
So what you should do is change your rules to;
On my MAC Rule 1 = C:\users\susie|/Volumes/B
On my other PC Rule 1 = C:\users\susie|E:
I just changed my rule from "G:\Music|\\WILMA\Music" to "G:|\\WILMA" and that worked. When I tried "G:
\|\\WILMA" it didn't work, so there is no intelligence in this functionality, other than substituting forward slashes and backslashes, which I think was already there. It just does a character substitution.