This a long shot but where about is your media saved? On a network share or external drive? If over the network are you connected via drive letter or UNC? Also what OS are you using?
The reason I ask is what you might be able to do as a work around (depending on your answers above) is copy your music library directly to your USB drive (rather than using RekordBox) and then if you're using a drive letter for your existing media, change the USB drive to be that same drive letter (so effectively RekordBox is reading from the same path)
I know that sounds like a cumbersome suggestion and but let me know if it doesn't make much sense and I'll clarify it a bit, (it's 9:30am here and I haven't had my second coffee yet)