Any updates on this issue?
I just got back from my first trip with a D800 and experienced the "hourglass" on a very intermittent basis. Sometimes 2-3 times per dive, sometimes not at all.
The camera is a late 2013 build with all updated firmware.
Cards were Sandisk Extreme SDHC-1, 32GB purchased from B&H photo. One is listed as 80MB/s, the other 45MB/S.
It did not seem to matter which card was used - both demonstrated this issue.