B757 Captain reported during cruise flight the ECIAS SPOILER message displayed. There was a violent vibration confirming at least one spoiler was extended. The pilots were given priority handling from ATC; ran the QRH; and made an uneventful landing.

Narrative

On ferry flight; the Engine Indicating and Crew Alerting System (EICAS) SPOILERS popped up at cruise at 4000 ft. At the same time; the First Officer (FO) Pilot Flying (PF) put in 5 degrees of rudder trim to control the roll. I ran the SPOILERS checklist in the Quick Reference Handbook (QRH). With the buffet and roll rate; we knew at least one spoiler had to be extended. Since we were an empty ferry flight; I quickly verified visually that the right inboard spoiler was raised. We [requested landing priority] with ZZZ approach and were almost on final because of the quick flight. I sent a maintenance write up and sent a message to Dispatch advising him. The FO continued to fly. With the gear down and fully configured; the buffet was on the verge of violent. We armed the speed brakes for landing since there was no prohibition in the checklist. We did not extend them manually during the flight. He landed without incident. Crash Fire Rescue verified we didn't have any leaks and we taxied to [the gate]. Both a mechanic and two Flight Managers met us for a de brief.