I installed Lr CC the night of April 21 (first day it was available) and have tried exporting several different photos, one at a time, without success; the pop-up message says "Some export operations were not performed. The file could not be written." Shutting down the program -- even restarting the computer -- had no effect. The photos are from different years, and thus are in different folders on the external drive where they reside. My computer is an iMac with Retina 5K display, Intel Quad-Core i5 processor, 16 GB RAM, and AMD Radeon R9 M290X graphics card with 2 GB of dedicated memory. Exported photos go to a separate folder within the year-month folder the RAW image is in; capacity is not an issue, as the 2 TB drive is less than half full (same drive and export arrangements as used with Lr 5 for about six months, without once encountering this export problem).
This is an update, not a reply. I set up a new external hard drive and put today's photos on it, then imported them into Lightroom CC. With the photos on the new drive, I can export photos -- so the problem may be with the old external drive.