I'm using an iMac with the latest OSX release and LR CC 2015.
If I select the highest level folder on the drive, and click show subfolders, then LR shows me all image files within that high-level folder with no subfolder structure. Selecting New images only will tell it to not show any images that it can find in the LR catalog. The term "new" has nothing to do with the date of the image file.
I also find that occasionally LR can't recognize images from e.g. an SD card that have already been imported into the catalog. When this happens, I Deselect All and then select the images I want to import. I'm not sure why this happens.
Hope this is helpful.