Use the "Audit" function in MAME (usually in the right-click menu or by pressing F5 in the file list) to verify which files are missing within umk3.zip . It will tell you exactly if 463_mk3_ultimate.u64 is missing or has the wrong checksum. Part 2: Solving "U64 Not Found" on Physical Arcade PCBs
Do not unzip the file. Simply drop the updated umk3.zip archive directly into your designated roms/ folder, allowing it to overwrite the older, broken version. Step 3: Check for Parent ROM Dependencies 463 mk3 ultimate.u64 not found