Part 2Making the linksIn order to use the Mountain Lion user folders from Snow Leopard, you need to link to them. I don't think it can be done with a standard Apple alias, you need to use a symbolic link. You can do that using the Terminal, but a far easier way is just to download this small application, , which installs as an item in the contextual menu. When you right-click on a folder or file the option is presented to make a symbolic link.
A symbolic link is somewhat different to a normal alias. It is a much smaller file and operates at system level instead of the GUI Finder level - it is a very short piece of code which contains the absolute address of the item it links to.
Mountain Lion home folder: UNIX symlink size compared to an Apple aliasSymbolicLinker generates the link directly and adds a 'symlink' suffix. You could rename it to anything you like, but in this case it's logical just to remove the suffix. It appears in the same directory as the folder of item it is linking to, all you need to do is move it to where you want it to go - into the user folder on the Snow Leopard partition. In this case as the destination is on another volume, it will be copied. Once that is done you can delete it's parent symlink, just throw it in the trash and empty it.
When you double click on the link or you drag it into the Finder sidebar or Dock, it appears just as though it is the original item in the Mountain Lion partition. If it refers to your documents folder for example, it opens the documents folder on Mountain Lion but it's just as though it's actually in Snow Leopard. To keep track of this it's very useful to enable the Finder window Pathbar from the Finder's view menu - it shows at the foot of the Finder window and it makes it easy to follow what's happening.
Because these links are absolute addresses, a clone that I have made of my Snow Leopard partition to an external hard drive behaves just the same as it does on the second partition, it connects directly to the Mountain Lion partition on the internal hard drive transparently. The symlinks were cloned as well.
This is very useful since I'm going to build a Fusion drive on my MacBook Pro, therefore I won't be able to use Snow Leopard any more on the computer directly. I've cloned the small Snow Leopard partition onto my Time Machine drive and it sits there quite happily next door to the Time Machine backup folder. The only issue is that I will run out of space on the disk sooner. I now use this disk both for my Time Machine backup, or can boot into it to run Snow Leopard with all links still functioning correctly.
ColourBecause the symbolic links connect directly to the folders in Mountain Lion they take the icon of those folders. As you know, ML has done away with the normal colourful icons that exist in Snow Leopard and replaced them with dull grey folders. I didn't like this so in order to regain the colourful items of the past, I copied the icons of the Desktop, Pictures, Music, Movies, Downloads etc. from Snow Leopard and pasted them onto the Mountain Lion folders in place of the grey icons. This means that these icons now also show up in the Finder window pathbar, the toolbar and some dialogue menus, not only in Snow Leopard but also now in Mountain Lion.
Here's a copy of the icons that you can use for the user folders on the Mountain Lion side. 'Get info' on the folder in question, copy and paste it's icon into the Info window for the corresponding ML folder.
The Finder sidebar in Mountain Lion is still muted, however, even though the folder icons themselves have been changed they still show up as the generic grey icons in the Finder sidebar because that is the way Apple have designed the new Finder. In order to change that you need to install a separate piece of software.
I've brought back the colour for Mountain Lion with SideEffects. It has an installer which takes care of everything, and it works very well. It even adds colour icons back to most (but not all) open and save dialogue boxes. These icons are optimised for either small or large view size, if you choose the medium size they are slightly blurry. The small size works best for me. System Prefs > General > Sidebar icon size.
While on the subject of colour, iTunes 10 can have it too by replacing the iTunes resource file. There are several different locations to find it on the net, and they mostly date from 2010. is one of them. Now if only somebody would fix the Mail icons. But at least they're not so important since there are only a few icons to distinguish.
I think there are more opportunities to use the symbolic linker. Many applications are common to both Mountain Lion and Snow Leopard so I'm sure you could use it to link apps too, increase disk space by throwing out duplicate applications. Be careful when linking to the Apple applications, since there are often different versions on the two sides. I've just deleted the EyeTV application from the Snow Leopard side and replaced it with a symlink - it still auto-boots on start-up and works normally. The external library is shared (schedules, recordings etc.), but it's preferences are local to the Snow Leopard user library.
Sharing media librariesI have only one iTunes library and one iPhoto library. If I used Aperture I would have only one of those. In order to do that I must ensure that the versions of the application are the same (at least same point size) on both sides. As you probably know Apple frequently change the organisation of those libraries with newer versions of the apps. I really don't know why they need to do this so often but anyway it's something to be aware of, to guard against having your library updated when you didn't want it to be. I didn't use symlinks for the media libraries - instead, hold the option key on application start-up - it asks you where the library is. Just navigate to it and choose it.
My start-up disk is set to Mountain Lion. When I need to use Snow Leopard I just use the option key at reboot to choose it temporarily.
I guess that's enough waffle,
I hope it will be useful to some people who might be considering doing this. It is a great way to benefit from the advantages of Mountain Lion (and there are many), yet still have the ability to run Snow Leopard when needed.
No more parts!