B747-400 flight crew reported a flaps drive anomaly in cruise resulting in a return to the departure airport.

Narrative

A normal departure from ZZZ took us through a normal level off; switch over to Center Controllers; and a crew change for the first break. I was on the first break; just over an hour after takeoff; the airplane was cruising at 32000 ft. and I felt a pronounced deceleration; it felt from my position on the upper deck as if the airplane was starting a descent. I went up to the flight deck and observed that we had a FLAPS DRIVE EICAS message and an expanded flap position display that showed the cross-hatch 'in transit' depiction for the right wing mid span leading edge device. I switched back into the left seat and while doing this I noticed the red maximum speed indicator on the airspeed had come down to 280 kts. while the red minimum speed indication had come up to about 255 and the roughly 25 kt. window in between was showing the minimum maneuvering speed amber line. The deceleration had been the autothrottle trying to reset the speed below the max speed for flaps 1 but also trying to stay above the min maneuvering speed and this was impossible because they were overlapping. It was close to 280 kts. which gave us a mach number of about .76 (we had been at mach .86 for cost index 600). I asked one crewmember to find a FLAPS DRIVE procedure in the QRH while I disconnected the autothrottles and set the speed bug to 280 at the bottom of the red. I asked the First Officer in the right seat to maintain the position of Pilot Flying and also explained to him he would have to maintain the speed manually. The QRH procedure takes you to a decision point asking whether FLAPS DRIVE appeared during flap retraction; or during flap extension; in our case neither of those cases applied to us so we could not accomplish much in the way of corrective action until reaching the deferred items and the instruction to use a Vref for flaps 30+25. At this point I confirmed with Dispatch and Maintenance Control via a satphone call that a return to ZZZ would be warranted. I initiated a re-clearance back to ZZZ via CPDLC with the Center Controller; which we received as direct ZZZZZ; direct ZZZZZ1 for the ZZZZZ arrival. Once on course back to ZZZ we requested; again through CPDLC; clearance to begin a fuel dump to reduce down to max landing weight; this was eased by the fact this airplane has the MLW option on the fuel jettison selector. We briefed that I would become flying pilot for the final approach and landing; and we had calculated our ref speed of 181 using vref 30+25 guidance from the QRH; and we also determined landing distances from the performance inflight section of the QRH. We had also asked the Dispatcher to do a landing distance calculation to back us up. I determined I wanted to use Runway XXL; the longest runway; and that we would plan a turnoff at the end with autobrakes 3 to prevent creating additional problems of hot brakes after landing at a higher ref speed. The Center Controller had asked us if we were declaring an emergency as did the Center controller when we got into radar contact. In both cases we replied negative. We started configuring flaps early as we had briefed; I had said I expected to get FLAPS PRIMARY and that the flaps would extend slowly. This is exactly what occurred; but as we got to flaps 1 and then flaps 5 we noticed that from the expanded flap display that all the flaps were indicating normally down. We also verified this visually out the window; that the midspan flaps were down on the right side. Further flap extension occurred at the normal speed although FLAPS PRIMARY remained displayed. Once we had the gear down and flaps 20 set; and all groups were in normal down position; I made the decision to use the normal flaps 25 ref speed for a slower landing since we were confident of the flap position. This resulted in a flaps 25 landing without incident; and the opportunity for an earlier turnoff with brake temp values 1 or less. We left the flaps at the 25 position for the taxi in.In this situation the crew workedtogether in excellent fashion; I believe this event can be a great example of good CRM and teamwork.