Official comment
@BS Help >I'm assuming these are Windows users and chances are the hard drive is connecting with a different drive letter, thus the original location within rekordbox is unavailable.
Please ask the users to map the drive to a letter (eg. X:\) using the Disk Management utility and then do a mass-relocate (select all tracks, right-click on the first missing track, select Relocate, find the file indicated at the top of the window, then when prompted, use the same location to find all other missing files).

You can see that one of them is mounted as PULSE 1, meaning to the system, it has a different path.