Air carrier ERJ-170 Technician reported they did not perform the MEL procedure properly for the engine starter control valve. Later troubleshooting determined that the engine gear box was seized.

Narrative

Myself and one other mechanic went out to Aircraft X after it returned to gate due to E1 no start. I was the more experienced mechanic but neither of us had performed a deferral of an engine starter control valve or a manual engine start. When we arrived at the aircraft we checked for faults on the CMC (Central Maintenance Computer) and found none. After speaking with the captain we opened the engine cowls to check the starter for damage. I have changed a broken starter before and this one had no signs of damage or failure so we thought to attempt a manual start after we closed the cowls. At this point I was reading the MEL 80-XX-XX-X and I did not understand that a full engine start was required to apply this MEL; With APU bleed on I manually actuated the starter control valve; I felt it turn and heard the rush of air; I could see that N1 began rotating so I assumed that the engine would start when the full manual start procedure was performed. I wrote up the deferral paperwork for the E1 starter control valve and signed it off then called it in to Maintenance Control to put the aircraft back in service. I coordinated with the captain and ground to perform the manual start and E1 still would not start after 2 attempts. I called Maintenance Control to take the aircraft back out of service and the aircraft was de-boarded. Later troubleshooting determined that the engine gear box was seized.