Official comment
First and foremost, rekordbox itself can read a variety of drive formats, but only FAT32 and HFS+ are usable by the players, so stick with those. As for partitions, players can only access the first one, and rekordbox is hit-and-miss, depending on how it's structured, etc.
There are some partition managers which can do non-destructive editing, but you really have to know what you're doing. My recommendation would be to make a backup of any data you have on the drive, then adjust it down to 1 partition, format it as needed, and dump the data back to it.