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I'm guessing you're using Windows and haven't assigned fixed drive letter assignments to each of the drives, so it's possible when you connect the drive, it receives a different drive letter than the previous use, and as such, rekordbox can't find the files.
Start by assigning the drives each a unique letter (try X or Z, for example), do one more relocation (it will then update everything to that new drive letter), and see how that works.