Rekordbox automatically deleting music off of usb

Q When plugging my usb into a friends laptop, I think I clicked on the "sync this device" popup when I initially plugged it in. Now, if I try to bring over new music, rekordbox automatically deletes any music that's on the USB, that's not already on his computer. Does anybody know what this "feature" is called, or how I can disable it? I've even tried formatting my USB and renaming it, and it still just wipes any new music that's on there as soon as its plugged in.