I am also having this problem, has it been resolved? I try exporting, with size limitation, to my D:/ drive, a 3TB internal drive. Half of the files give the "some export operations were not performed" error. When I look at the folder in explorer, the ones with the problem show a generic icon instead of a thumbnail. Clicking on one of them gives me a message that the files are being used by LR. When I exit LR, the images are fully available, just larger than I had requested
If I switch the destination to my C:/ SSD, it works fine. The files that didn't have issues are exactly the same size. The files that had problems are larger on the D drive. I updated to the latest 4.4 but this apparently isn't one of the bugs that was fixed. Having to export to a drive other then where all of my images are stored, in a very structured naming convention/folder hierarchy, is not an acceptable permanent workaround.