Could you explain in more detail?
Also, please tell us how big these photos are in pixels (width and height), and how much memory your computer has, and what operating system are you using?
ffull size canon mark iii 5d files exporting at quality 90 resolution 300
imac osx Yosemite 10.10.3 8gb memory.
EXporting to. WD external drive, less than half full. As I usually have done with no issues.