Official comment
That sucks, sorry to hear it.
When you connect the USB drive, it will appear under Devices - not as your main collection. First thing to do is create a new playlist on the device, call it All Exported. Go to All Tracks on the USB, then select all the tracks (CTRL+A on PC, Command+A on Mac), right-click on any one of them and select Add to Playlist > All Exported. Next, right-click on the All Exported playlist and select Import Playlist. It will prompt you for a destination, telling you the number of files and amount of space it will import. Click Copy then sit back and wait.
All those songs complete with grids, memory points, hot cues, etc. will be added to your main rekordbox collection.