In the Commander mode window of the D800, what Group and Channel are selected for the remote SB900?
Is the remote SB900 set to "Remote" mode, and the same group and channel as selected in the D800?
Is the SB900 firing in response to the preflash pulses emitted by the D800 popup flash?
Is your flash sync mode set to "Normal", the little lightning bolt symbol in the flash mode window?
Set your flash mode to rear curtain, and then select a two second shutter time. You should see a preflash pulse when you push the shutter button all the way down, then the remote SB900 should respond. Then you should see a low power "fire now" pulse from the popup flash just before the shutter closes, and the remote SB900 should fire with it.
HBB in Phoenix, Arizona Nikonian Team Member
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