I'm currently helping a friend with the same problem. She has a Sandisk 32GB microSD card that she used fine in her older Samsung S3 phone which contained thousands of pics. When she got her new Samsung Note 3 phone, she put in the same SD card from the S3 to the Note 3 phone. At first things worked fine for a few weeks, then suddenly one day all her old pictures were gone, except for a few dozen recent pictures taken. Weird.
When I tried placing her SD card into the micro-SD card adapter and inserted into a laptop, the laptop could not find any of the older pictures in the D drive. I had not done any formatting/reformatting whatsoever for fear of erasing anything. I then went to CNET's site of
Free software downloads and software reviews - CNET Download.com and installed a free Windows app called "Pandora Recovery". I specified doing a search in the D drive where the SD card is located, and after 30 minutes, the app had already found over 1,500 JPEG files. Because the search required longer time, I left yesterday before it finished. I will go back later today to see the outcome of the recovery. Hopefully the app will be able to recover the "lost" pictures.