I thought I had this figured out but it appears to occur somewhat randomly. Since upgrading to Windows 8 when I export files that I have imported from Canon raw .CR2 for some files it gives an error that "the file could not be written" and lists the original source .CR2 file name. It does not do it everytime but once it happens it happens everytime for that file. When exporting 10 JPG files for example only 3 of them may have this issue. The file still exports and creates the JPG file and the only issue with the exported file appears to be that it did not create the preview thumbnail.
I have tried importing the files into new catalogs, writing to different folders etc but it still happens. Any ideas?
Ok, after lots of attempts to recreate I can do it like this:
Export as JPG to hard drive and select 'limit file size' option and give any size smaller than the full 100% quality JPG would normally create. Using any other options to limit file size such as quality < 100 or resize long edge to 1024 completes successfully.
Can someone else confirm this on Windows 8 with LR 4.2, it only takes a single file?