>Every 10 shots or so I get very slow writing speed only >because there is a SD card in the second slot. I am NOT >writing to this card but it is there just in case I forget CF >cards at home (happened before).
Consider making the problem go away by removing the SD card and storing it in your camera bag.
>So, at CES I talked to Nikon rep who supposedly is in service >department. >I was told that because SD card is part of the bus now it does >not matter if I am writing to it or not. It slows down the >whole bus.
The menu setting to avoid the SD card does not turn off the slot's electronics. Any card in the slot still has to be part of the system even if data is not actually being written to it. That said, I'm personally not sure about the Nikon rep's response.
>Except it does not happen every shot. And it happens >randomly.
. . . which is another reason to suspect some other culprit. If you're using a battery grip, you may accidentally be bumping its shutter button in a half-press, which can activate the buffer and slow things down. A third-party/non-Nikon grip can cause problems too.
>He suggested to use the same speed SD card as CF card but I >don't have 400x SD cards.
Personally, I think a 36mp monster like the D800 absolutely needs the best and fastest cards in order to prevent the camera from drawing unnecessary attention - to prevent it from distracting us.