A number of files of various formats (avi, MOV, mp4, NEF, jpeg, tif) will not import into Lightroom 5.6, although Elements 12, and Photoviewer have no problem dealing with them. I don't think the files are corrupted and the same hardware is used in each case importing from an external USB3 disk. When importing under the 'Add' option most files import without a problem but some fail with the message 'An unknown error has occurred while reading the video file. Connecting to Dynamic Link server failed', and others fail with the message 'no photos found for import'. I am new to Lightroom and would really appreciate some help with this, R.