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I'm add a additional index on a table,
but after a few minutes I get the following error message: And the index is not placed.

I check the partition, but there is still more as enough free space.

The error message says that your temporary file-system is full, or has a problem.

Usual it is /tmp/

The destination for the temporary directory is specified in the my.cnf file under the variable tmpdir.
The my.cnf is usually stored in /etc/mysql/my.cnf

Try setting to another partition?

You can find the tmpdir with the command: if you change it in /etc/mysql/my.cnf,
you need to restart the mysql-service.