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 should create a symbolic link between the two folders (saves and the saves folder on your USB drive).
Instructions on doing this with your Minecraft worlds are shown at http://minecraft.gamepedia.com/Tutorials/Saved_data_Dropbox_guide_saves_only. Instead of using dropbox as your destination folder however, you can use a folder on your USB drive. The quote below from the Minecraft Wiki (link above) shows you how to do this on Mac OS using the terminal.
ln -s ~/Dropbox/Minecraft/saves ~/Library/Application\ Support/minecraft/saves
Replace ~/Dropbox/Minecraft/saves with the destination folder.
Best Answer
They can be accessed by other realm members, but unless they've been given manager privileges, all they can do is join the realm server. Without access to the management interface, they can't download the world^.
^There's other ways, but we won't get into that.
Of course it wouldn't work. You can't drag a server instance onto a USB drive; that'd be absurd.
Follow the instructions on screen, and Minecraft will save a copy to your Single Player worlds.
From there, you can copy it out of your Single Player worlds folder and into wherever you want.
On Windows, it is located at:
%AppData%\.minecraft\saves,On Mac, it is located at:
~/Library/Application Support/minecraft/saves, andOn Linux it is located at:
~/.minecraft/saves(assuming~/is your home directory)You'll find that the downloaded world retains all Multiplayer data (sans ownership permissions), so you can simply plug this world into a dedicated server and it'll remember all your friends' progresses.
Images stolen from here