Kbase Article of the Week: Mac mini: How to Identify External Power Adapters on Mac Mini Models Released Before 2010 »

Apple Support:

In February 2006, Apple introduced the Intel-based Mac mini (Early 2006). This model and models of Mac mini through 2009 use a different power adapter than models of Mac mini released before 2006.

The two power adapters are not interchangeable, and should only be used with the model of Mac mini they shipped with. Read on to learn how to identify which power adapter to use with each Mac mini computer.

When Apple redesigned the Mac mini in 2010, the machines went to internal power supplies.