I noticed a thread on the old discussion board about an issue the D5100 had with the SanDisk SDHC 16GB card. It appeared to be resolved for that camera with a firmware update.
I wonder if it is coincidental that I have just had a problem with reading the same SanDisk SDHC card in my laptop's card reader. I at first blamed the 8 in 1 cardreader (from ASUS) which had no trouble reading other media including the regular SD cards, however, there are lots of posts out there about needing to update drivers in Windows to read the high capacity, but, the card reader after checking all of that should have been able to read the card.
I was able to download pictures using the USB cable, that's just not my preferred method.
I narrowed it down to the cards themselves (yes I had two of the SanDisk cards) when I plugged in a Kingston SDHC card I had lying around...it worked perfectly in the camera and the card reader. I tried formatting the SanDisk cards both in camera and using the computer (through the USB of course) with no change in the issue with reading the card.
I have reported the issue to SanDisk, Asus and now here just in case it helps someone else