Kevin, you are going to need to give us more info that this.
1) What have you tried to do to correct the situation?
2) What exactly are you doing (be very clear becasue otherwise we make assumptions and get things all wrong and fail to give you good quick advice)?
3) What gave you the error message, FreeNAS?
4) And please follow our forum rules and post your system specs.
Based on what you did provide my first suggestion is to try to copy the suspect files to a hard drive, not an SD card. SD cards can be flaky at times. So first ensure it's an actual FreeNAS issue and not something else.