A B777 pilot reported a cargo door open warning during cruise. Flight returned to departure airport.

Narrative

Just after level off @ FL330; approximately 40min into flight; the DOOR FWD CARGO EICAS Caution illuminated. At Captain's direction and as PM; I immediately accessed the electronic-checklist; on the lower Control Display Panel (CDU); while pulling up the DOOR SYNOPTIC on my Multi-function Flight Display (MFD). DOOR SYNOPTIC indicated the FWD CARGO DOOR open; with no indication of depressurization or airframe vibration (ever). Captain transferred aircraft control to me; then requested priority handling to ZZZ [ARTCC] and asked for/cleared for a descent to 8000 feet (per checklist) and a turn back toward ZZZ1; initially given direct; w/ subsequent routing provided later. Captain called the Relief Pilot (newly settled in the crew rest bunk) to return forward. Captain contacted Dispatch and Maintenance Control to inform them of our situation and pending return. He then briefed crew and passengers to inform all of the plan. We agreed to dump fuel given the gusty crosswinds at ZZZ1. I suspended the DOOR FWD CARGO electronic-checklist since we could not complete until level @ 8;000.' Per Captain; I began the Fuel Jettison Checklist. We subsequently dumped down to landing weight; completing fuel dump about 75 miles North of ZZZ1; then completed the remaining items in DOOR FWD CARGO checklist & the DESCENT checklist. Step 1 of DESCENT Checklist; Recall & Notes; did not indicate any additional concerns/considerations from the EICAS or the non-normal checklist notes. While cruising at 8000 feet DOOR FWD CARGO EICAS Caution went away; as did the open indication on the DOOR SYNOPTIC. The remainder of arrival/approach was uneventful. When queried by ZZZ1 [Tower] Controller; we told them we'd like to clear the runway and return to the gate XX; w/ no assistance required. NOTE; auto speed brake did not deploy initially upon touchdown/in ground mode. I watched it for ~2 extra seconds. As I was preparing to verbalize 'speed brake;' the handle finally tracked aft to the proper position. Autobrakes appeared to work normally. We did not have any trouble w/ deceleration or stopping the aircraft. Upon clearing the runway XXR and crossing runway XXL; I accomplished After Landing Flow; verified completion w/ electronic-checklist then pulled up the Status Page; curious to see if additional messages appeared on landing in light of the delayed auto speed brake deployment. Status Page indicated the following messages: 1) AUTO SPEEDBRAKE; 2) GEAR INDICATION SYS; & 3) PASSENGER ADDRESS. Upon completion of PARKING Checklist & debrief; we discussed whether to continue or not and agreed fatigue is an issue and would get worse w/ an uncertain timeline. All agreed to not continue. Operations Rep; Chief Pilot (Crew Scheduling through Chief Pilot) and Maintenance personnel were extremely responsive after we opened the aircraft door. Overall; the event was relatively straight-forward and we brought it to a successful conclusion w/ effective communication and CRM; along w/ great support from all parties.