Are there any adapters or "hacks" that make it possible to connect a second external display to a MacBook, MacBook Pro, or MacBook Air model?

By default, all MacBook, MacBook Pro, and MacBook Air models are capable of simultaneously supporting the internal display at its native resolution and a single external display either in "mirroring mode" -- where the contents of the internal display are duplicated on the external display -- or in "extended desktop/dual display mode" -- where each display is treated as additional work space.

However, it is possible to attach a second external display with a USB 2.0 to VGA or DVI adapter. There are a variety of these adapters available, but four guaranteed to be Mac compatible and support a maximum resolution of 1600x1200 or 1680x1050 include the Gefen USB to DVI Graphics Adapter, EVGA UVPlus+16 Adapter, OWC USB to DVI/HDMI/VGA to Video Display Adapter, and VillageTronic ViBook.

In basic terms, they all work in essentially the same way by compressing and decompressing the video signal in order to utilize the limited bandwidth provided by USB 2.0, each cost in the neighborhood of US$100, and all make it possible to support an additional external display on any Intel-based Mac running MacOS X 10.4 "Tiger" or MacOS X 10.5 "Leopard". These are not compatible with PowerPC-based Macs.

For the MacBook Air models (which have one USB 2.0 port), you could connect a second external display with a single adapter, on the 13-Inch and 15-Inch MacBook and MacBook Pro models you could connect a second and third external display with two adapters (using both USB 2.0 ports), and for the 17-Inch MacBook Pro models you could use all three USB 2.0 ports and connect as many as three additional external displays with three adapters.

Being able to connect additional displays to a MacBook, MacBook Pro, or MacBook Air sounds fantastic, but it still is important to be aware of the limitations of the technology. The bandwidth provided by USB 2.0 is insufficient to "fully support" OpenGL 3D hardware acceleration, and as a result, there is a "lag time".

Consequently, these adapters do not work well with gaming or other applications that place a great deal of emphasis on graphics performance such as high-resolution video playback or video editing. However, the lag time is unlikely to be bothersome for basic productivity -- word processing, spreadsheets, web browsing, chat, and so on. It is quite possible to use more graphics intensive applications on the "main" external display (connected by the DVI, mini-DVI, micro-DVI, or Mini DisplayPort, depending on the model) and less graphics intensive applications on a display connected via the USB to DVI/VGA adapter.

In a fantastic review of the Gefen product that absolutely should be read in its entirety, MetkuMods provides more technical information about how these adapters work and real-world performance details, including this helpful video demonstration of the lag:

Site sponsor Other World Computing's affordable USB to DVI display adapter is part of a kit that includes a DVI to VGA adapter, an DVI to HDMI "swivel" adapter, and the necessary drivers.