MySQL backup breakes a table every time

I've a MySQL Server 5.1.58-1ubuntu1 installed on an Ubuntu 12.04 LTS.

Every night we do a backup of all the databases with this script:

rm -rf /var/backup/mysql/ *for i in $(echo 'SHOW DATABASES;' | mysql -u root -p'password' |grep -v '^Database$'); domysqldump \-u root -p'password' \-Q -c -C --add-drop-table --add-locks --quick --lock-tables \$i > /var/backup/mysql/$i.sql;tar -cpzf /var/backup/mysql/$i.sql.tar.gz /var/backup/mysql/$i.sql;rm -rf /var/backup/mysql/$i.sqldone;

Every time the backup is executed a particular MySQL table is broken (the MYD and MYI files are deleted, while the FRM remains).

We have many databases and tables, only this one is broken.

Any idea?