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The disk mode is just the apple firmware. Reboot into Apple's firmware and then plug in USB.
You should be able to drag and drop files anyway. The screen that says "Multimedia Mode" is the Rockbox USB mode, which should work fine as a regular mass storage device. The "Multimedia Mode" text refers to what keyboard keys the player's buttons will simulate: this is for USB HID, a feature unrelated to file transfers.
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