Tracks won't load

I have a weird but very annoying problem...

I have several usb devices...
1x SanDisk Extreme USB 3.0 128 gb device
1x SanDisk Extreme USB 3.0 64 gb device
1x SanDisk PRO USB 3.1 256 gb device.

I have no problem exporting mp3 files to the smaller devices, but rekordbox won't load a lot of my files to the 256 stick. I have bought a second 256 gb stick, similar to the one I already have, but the same problem occurs so it is not a corrupt usb device.

I tried uploading tracks from Rekordbox from another computer, but I get the same problem. I get an export log that states a list of the files that are not uploaded.

I made different changes to one of the mp3 files, changed the bitrate, created a new mp3 file, and so on, but all with no success.

What could be the problem?