Try copying all the data from your SD card to your pc, then reformating it with
SDFormater, then copying the data back and trying again. When i went from 10.1 to 11 I had serious problems with my SD cards. I had to reformat both the card I used to install CM and the card I left in my Nook HD+ to store backups to before the Nook would work with them. On the install card it was so bad the Nook not only wouldn't boot, it would require a hard power cycle to get it to boot up again even with the card removed. Both cards worked fine under 10.1.
CM 11 seems to make the Nook HD+ pickier about SD cards somehow.