Mode Not Supported means that there is a signal resolution that the television doesn't recognize or accept. The television will accept HDMI (720p, 1080i, 1080p) signals, so perhaps there is a way to upconvert the standard definition programming in the box that isn't being done. Is there a way to block 480i/p signals or force an upconversion in the box itself?
It may require a call to DirecTV's customer service, but they may be able to guide you through the menu system in the box and fix the issue.
Keep me posted.
I recently purchased a LN46B750. I am also a Directv customer. I have an issue turning my set on when I am on a channel that is being broadcast in anything other than 1080i. I receive a "mode not supported" message. The only way around it is to select a function on the Directv remote, such as "menu" or "channel up"? At that point, the picture will show up.
As mentioned, I have no issues turning on the TV when it is on a channel being broadcast in 1080. I have set my Directv box set for 720 and 1080 formats. I am using a Monster 1000 cable.
I have tried multiple HDMI ports and cables with the same result. If anyone could help me, I would be very grateful.