You could probably make it recursive to search for aliases in selected folders, but that's more work than I care to do - and then there's the problem of dealing with aliases to folders. The following applescript will take selected aliases in the Finder and try and relink them to the new path replacing Backup with External in the POSIX path. OS X being a hybrid of the Mac and Unix worlds causes some interesting entanglements when it comes to keeping track of the locations of things and their aliases. Details: In Mac OS X, an alias is a pointer file that allows you to quickly open the files. Simply select a file, folder, application, or whatnot and select File Make Alias or press -L (in OS 9 it was -M, which now, sadly, minimizes the current window instead).
Here's my stab at solving this problem with Applescript. What are Smart Folders on Mac and How to Use them - TechPP. When you create an alias in OS X (File -> New Alias or Command-L), the alias’ icon displays with a small black arrow in the lower left hand corner.