Minecraft – How to get to the .minecraft folder without the %appdata% short-hand

I understand that you have modified Minecraft so that it writes onto the USB drive, but not in the same place for each version. All you need is a symlink to make the Mac side do what you want.

First, delete the Mac side's saves, then in a shell, run the following (based on your comment mentioning the path for Mac; adjust the path to the Windows side as needed):

cd '/Volumes/myusb/Library/Application Support/Minecraft'
ln -s '../../../whatever-the-path-is-to-the-windows-side/saves' ./saves

If I haven't got the Mac-side path right, then make sure the count of ..s is adjusted to match. To test your symlink, run ls saves and you should see your list of worlds.

Most applications, including Minecraft, will follow a symlink as if it is not there, so this should do the trick. (Note that when viewed by the Finder, a symlink appears identical to an alias, but an alias will not function as needed here. An alias is more like a Windows shortcut — it is not automatically traversed by file system calls.)

You can't move the folder away from where Minecraft keeps it – Minecraft won't know where you put it and will just assume you've deleted your saves.

You need to put it back in your "minecraft folder" which is in [home]/Library/Application Support/minecraft/.

However, you probably can't find that location easily because Library is a hidden folder. The easiest way to find it then is to open Minecraft, log in, click Options… → Resource Packs → Open resource packs folder. This will open a Finder window showing you a (probably empty) folder. With this Finder window at the front, press ⌘+↑. That will navigate up into your Minecraft folder. Put your saves folder here, so that there is the minecraft folder with a saves folder inside it, with your worlds inside that.

If you want to be able to find your saves easily still, right-click the saves folder and choose "alias". This will make a shortcut that you can move anywhere you want.