Rekordbox export mp3's

I have about 19.000 tracks on my laptop that I want to upload on my USB.

I deleted all the mp3's I had on a USB stick so I could re-export the tracks on this same USB.
After I upload my tracks to the USB I get a log file that several (about 1.500) MP3's have not been uploaded.
I have no idea why!

I formated the USB and tried again, but still, not all tracks are uploaded.
If I try another USB the particular tracks load on the USB with no problem.

Does any body have an idea of how to solve this?