Starting MySQL.. ERROR! The server quit without updating PID file (/usr/local/mysql/data/backup.pid).

The error message suggests that the MySQL server was not able to start properly and update the PID (process identifier) file. There could be several reasons for this issue. Here are a few steps you can try to troubleshoot and resolve the problem:

1. Check the error log: Look for the MySQL error log file, which is usually located in the MySQL data directory. The error log might provide more specific information about the issue.

2. Verify permissions: Ensure that the user running the MySQL server has proper permissions to access and write to the necessary directories and files. The MySQL data directory and PID file should have appropriate permissions for the server to update the PID file.

3. Free up disk space: Check if there is enough available disk space on the server. If the disk is full, MySQL may fail to start or update the PID file. Free up some space if needed.

4. Restart the server: Try restarting your server and see if it resolves the issue. Use the appropriate command to start MySQL based on your operating system.

5. Remove the PID file manually: If the error persists, you can try removing the PID file manually. Locate the PID file mentioned in the error message (/usr/local/mysql/data/backup.pid) and delete it. Then, try starting MySQL again.

If none of these steps resolve the problem, it might be necessary to seek further assistance from a system administrator or consult MySQL support resources for more specific guidance.