MacOS – Smart folder using “or” search

You can access the partition folders from /Volumes (as mentioned below). To create the folder, you can do so by creating an Automator Folder Action such as this one:

Create a new Automator Folder Action and set the path to the Applications folder of the Volume. Then, drag the "Get Folder Contents" action in, and check "Repeat for each subfolder found." Then, drag in a "Copy Finder Items" action, and modify the path to your choosing. Check "Replace existing files" so that if you have duplicate applications in each of your /Applications folder you won't get an error. NOTE: this will override any application data.

I created this one, but all you should have to do is to clone it three times (for each of the partition folders) and then modify the paths. You can also change whether the apps should overwrite other apps. I hope that this answers the question better than my other answer! I'll see if I can come up with some better solutions. Also, I don't think that you will be able to filter the apps based on the OS X version.

By default, the Spotlight GUI in the Finder will not let you use wildcards or other advanced syntax. However, by using the Raw Query search criterion, you can use all the syntax available to the command-line mdfind utility:

• Navigate to your ~/Music folder
• Hit ⌘ F
• Double-check the Scope Bar of the search to make sure Spotlight is just searching your Music folder
• Hit the "+" button to add a search criterion
• From the criteria drop-down menu, check if the Raw Query criterion is visible (it isn't by default). If not, click Other... and a criteria list will appear. Search for the Raw Query criterion, check the box to include it in the menu, and click "OK".

• Select the newly-visible Raw Query criterion and enter the following:

  (kMDItemFSName == '*.mp3') || (kMDItemFSName == '*.ogg') || (kMDItemFSName == '*.flac')
  

The above query will search for files that have .mp3, .ogg, or .flac in their filename (as opposed to the contents of the file). This Macworld article helped me construct this particular query, but this Apple Developer page has a more comprehensive listing of the types of metadata available for raw queries.

Alternatively, you can construct the desired query visually:

• Navigate to your ~/Music folder
• Hit ⌘ F
• Double-check the Scope Bar of the search to make sure Spotlight is just searching your Music folder
• Hit the "+" button to add a search criterion
• Choose the Kind criterion and set it to Any
• Hold down the Option/Alt key to create a conditional (thanks to Adam Franco for pointing this out) and set it to Any
• From the criteria drop-down menu, check if the File extension criterion is visible. If not, click Other... and a criteria list will appear. Search for the File extension criterion, check the box to include it in the menu, and click "OK".
• Add as many File extension criteria as you wish.

If you want the search to display all files below a given folder (regardless of file type), you can use the File visibility criterion:

• Navigate to your ~/Music folder
• Hit ⌘ F
• Double-check the Scope Bar of the search to make sure Spotlight is just searching your Music folder
• Hit the "+" button to add a search criterion
• From the criteria drop-down menu, check if the File visibility criterion is visible (it isn't by default). If not, click Other... and a criteria list will appear. Search for the File invisible criterion, check the box to include it in the menu, and click "OK".
• The File visibility option should now be in the search criterion drop-down menu. So long as this criterion is set to "Visible or Invisible", all files below the present folder will be shown (you can optionally change this to "Visible Items" you don't want dot-files in the results)