Does it make any difference if you have "Show subfolders" toggled on/off?
..but then it probably needs it on, otherwise it would not find anything...?
The only reason this does not bother me is probably the difference in workflow: I use LR's import to copy the images from my camera cards and create the desired subfolders inside "My Pictures". Then they stay there, not getting any more filled other than by maybe a 2nd LR-import.
The drawback is that I can trigger only 1 import from 1 card at a time.
So I can understand a different choice in workflow, where you parallelize the copying from multiple cards via OS and then just trigger one synchronize folder as import action.
It doesn't make any difference with show subfolders toggled off. In fact, it takes an awful long time for the task to complete, with the end result being the same. I have also tried show files in the synchronise dialog, which showed me the last of missing files. When I then clicked import, the task doesn't complete. So I assume that it doesn't like those last 11 files. They are a mixture of jpg's and psd's. Have to now check what the problem is.
I have different folders for different cameras. I don't use LR to copy from the cameras, but their relative programs. I then copy them to my working folder. So LR has to find the pictures in those folders as well
well, after spending a long time opening, importing and closing LR4, found the actual problem. It came down to the last 11 photos. It took me a while to wake up to the fact that LR said it found 6 new photos to import, but then found 11 in the import screen. So used method of elimination, by importing just one at a time, till it came to one that caused a crash. Wrote these down and found that they already existed in the catalog. So in the end removed them an reimported them. Bingo - no more crashes, no more photos found.
Now I just have to see how it goes at the next new batch, because on of these was a recent photo