MORE UPDATE: There’s been reports that while this guide worked in getting the drivers loaded for the external Bluetooth adapter, some was still stuck with the Apple’s built-in adapter. UPDATE: Looks like with OS X Mountain Lion, you don’t have to do the following hack anymore! The Broadcom dongle I describe in this article is supported on OS X Mountain Lion by default! In fact, the new kext includes support specifically for the IOGEAR GBU521 Bluetooth 4.0 Micro Adapter ! Also, you might want to consider getting the IOGEAR one since the eBay one I got has a bright blue LED that’s always lit and can be pretty annoying at night.
