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If you have not exported with 4.0.6 or 4.0.7, the first time you do will be significantly longer than normal as rekordbox is adding the additional data to the USB device for compatibility with the CDJ-2000NXS2 system.
If your export is hanging, it is likely due to an operating system error as rekordbox uses system functions for that process, it is not handled by rekordbox itself. Check your USB drives for errors and/or reformat them if necessary.