Keeping your camera firmware current is very important.  Nikon recently made two firmware updates available to customers D750 C:1.01 (download)

• When shooting with an optional Speedlight and Auto FP high-speed sync enabled with On selected for ISO sensitivity settings > Auto ISO sensitivity control in the photo shooting menu, images were sometimes over-exposed. This issue has been resolved. (Enabling auto FP high-speed sync: Select 1/200s (Auto FP) or 1/250s (Auto FP) for Custom Setting e1: Flash sync speed)
• When menus were displayed with the camera connected to a 4K-compatible TV via HDMI, display in both the camera monitor and on the TV was not correct. This issue has been resolved.
• Noise that could sometimes be heard when Custom Setting d1: Beep was set to any option other than Off has been reduced.

D810 C:1.02 (download)

• When playback zoom was applied to an image displayed in the camera monitor (image display enlarged) while the memory card access lamp was still lit immediately after capture, the image was not correctly displayed. This issue has been resolved.
• When menus were displayed with the camera connected to a 4K-compatible TV via HDMI, display in both the camera monitor and on the TV was not correct. This issue has been resolved.
• Noise that could sometimes be heard when Custom Setting d1: Beep was set to any option other than Off has been reduced.

Info courtesy Nikon