check your permissions for that macbook
Every file and folder on a Mac OS X hard drive has a set of permissions —settings that determine which user(s) have access to each item, and exactly what that access is. For example, permissions dictate whether or not a particular user can open and edit a particular file. But permissions also determine which items the operating system—or specific parts of it—can access and modify, and which files are accessible by applications.
http://support.apple.com/kb/HT2963
If that fails:
offload all data from the WD to a safe place.
format the WD in disk utility as MAC OSX extended journaled
All WD drives, alas, should be wiped clean of their resident spamware 'applications'