Have you looked at the log file as recommended? It gives reasons for each failed export.
Tracks not Syncing with USB (log file)
Strangely I have a new macbook air, and on my recordbox i have 1000's of tracks (via playlists in iTunes)
When I try and sync the tracks to my USB, i consistently get a message saying
'ONE OR MORE FILES WERE NOT IMPORTED - CHECK THE LOG FILE SAVED IN THE 'DOCUMENTS' FOLDER - EXPORT LOG - ETC
Can anybody shine a light on why these files are not syncing? Its an issue i encounted every time i upload tracks / I v rarely had this issue on my old laptop - maybe 1/500 tracks, but on the new laptop its 30-50%
Yes it says
track(s) were not exported due to the following possible reasons.
 - The file doesn't exist.
 - The file is in the format that CDJ/XDJ/mobile device doesn't support.
 - The file could not be copied.
 - Track analysis data is corrupted or doesn't exist.
 - Error occurred in the USB/SD/mobile device.
Hard to get my heads around what the actual issue us - this is only an issue on this laptop and not on my old laptop even with the same files. Not sure why the file could not be copied. I have bought a new usb to trial and see if this might help with the issue
@Charlie > In front of each file that did not export, one of those numbers is there for each error. I'm familiar with the list of error reasons, I was more curious about the specific errors you were getting.