Please refer the below related discussion.
If the above link doesn't help, Install the latest version of DNG Converter 9.3 and try using it.
Let us know if that helps.
If your destination folder for the converted DNG files is also on the camera card where the originals are, the card may be full and will not accept more images. Make the destination the hard-drive. Or copy the originals to the hard-drive before running the conversion.
Thanks everyone! I was sure that when I set up converter I had specified that the converted files be put on my hard drive but after reading your posts I discovered that I had selected Same Source. Which meant that Converter was trying to write back to an almost full card. Problem solved and thanks so much for bailing me out of what could have been a really bad situation.
Wow! Thank you, this just solved my problem.