Random Issues with Exporting Playlists to USB

I get the following error in the log after exporting to (a brand new - or old) USB:

One or more files were not exported. 
Track(s) were not exported due to the following possible reasons.
[1] - The file doesn't exist.
[2] - The file is in the format that CDJ/XDJ/mobile device doesn't support.
[3] - The file could not be copied.
[4] - Track analysis data is corrupted or doesn't exist.
[5] - Error occurred in the USB/SD/mobile device.

Each time I wipe the USB, different tracks don't export compared to the previous attempt. Sometimes it says a track didn't export - and it is playable on my XDJ-RX. After lots of hours spent on this in the week, I turned up at my most important gig - and my main USB would show the tracks, but I got a load error on both CDJs and an XDJ-XZ! Luckily my backup had most of the tracks intact. Never had so much trouble in all the years of exporting tracks (except occasionally when a file has changed location). I suspect this to be due to latest Rekordbox update - but see nothing on this in recent forums. Please help (soon!) as I have another important gig this Friday, supporting a well-known UK ska band!