Where are partitions defined in mysql
There is a CREATE TABLE syntax where you can declare the data folder and index folder under the partition_definition of a partition_option:

partition_options:
    PARTITION BY
        { [LINEAR] HASH(expr)
        | [LINEAR] KEY(column_list)
        | RANGE{(expr) | COLUMNS(column_list)}
        | LIST{(expr) | COLUMNS(column_list)} }
    [PARTITIONS num]
    [SUBPARTITION BY
        { [LINEAR] HASH(expr)
        | [LINEAR] KEY(column_list) }
      [SUBPARTITIONS num]
    ]
    [(partition_definition [, partition_definition] ...)]

partition_definition:
    PARTITION partition_name
        [VALUES 
            {LESS THAN {(expr | value_list) | MAXVALUE} 
            | 
            IN (value_list)}]
        [[STORAGE] ENGINE [=] engine_name]
        [COMMENT [=] 'comment_text' ]
        [DATA DIRECTORY [=] 'data_dir'] &lt;&lt;&lt;&lt;&lt;---------
        [INDEX DIRECTORY [=] 'index_dir'] &lt;&lt;&lt;&lt;&lt;---------
        [MAX_ROWS [=] max_number_of_rows]
        [MIN_ROWS [=] min_number_of_rows]
        [TABLESPACE [=] tablespace_name]
        [NODEGROUP [=] node_group_id]
        [(subpartition_definition [, subpartition_definition] ...)]


CAVEATS


I know it works for MyISAM because I wrote about it in September 2011 : Dynamically changeable database. For an already existing MyISAM table, it can done from the OS by moving the .MYD and .MYI to another disk and setting up two symlinks. I wrote about that in May 2012 : Can I have one table be an &quot;alias&quot;, or &quot;symlink&quot;, to another?. When done correctly, SHOW CREATE TABLE will display DATA DIRECTORY and INDEX DIRECTORY and their respective folders.
I personally implemented this in production for a client when I did the following


Converted mysql.general_log from CSV to MyISAM
Moved /var/lib/mysql/mysql/general_log.MYD to a large 3TB disk
Moved /var/lib/mysql/mysql/general_log.MYI to a large 3TB disk
Setup /var/lib/mysql/mysql/general_log.MYD as a symlink to the real .MYD
Setup /var/lib/mysql/mysql/general_log.MYI as a symlink to the real .MYI
enabled the general log
Watched the client's general log growth to 1.1TB
I wrote up how to set this up back in June 2011 : Mail alert from MySQL when databases are edited (under Option 3 : You Could Monitor the general log)

Using hard links, it can be done to MyISAM tables in Windows, although the effort to do so may not be worth it. I wrote about that back in January 2012 : Can I specify a different disk for a specific MySQL table?
I do not believe [DATA DIRECTORY [=] 'data_dir'] and [INDEX DIRECTORY [=] 'index_dir'] were implemented for InnoDB. Even if the it were implemented for InnoDB, I would adamently advised against it and I wrote about why you should not back in August 2011 : MySQL InnoDB data file directly on a disk slice (fixed-size)?


Your Original Question

MySQL 5.5 Only : With regard to your question in the heading, Where are partitions defined in mysql, you can run this query to locate all tables that have partitions:

select count(1) partition_count,table_schema,table_name
from information_schema.partitions
group by table_schema,table_name
having count(1) &gt; 1;


Here is a sample:

mysql&gt; select count(1) partition_count,table_schema,table_name
    -&gt; from information_schema.partitions
    -&gt; group by table_schema,table_name
    -&gt; having count(1) &gt; 1;
+-----------------+--------------+-------------------------+
| partition_count | table_schema | table_name              |
+-----------------+--------------+-------------------------+
|              31 | clickit      | t_keyword_conversion_ga |
|               2 | part         | myoldpart               |
|               3 | test         | messages                |
|             256 | test         | ngram_key               |
|               2 | test         | rc1                     |
+-----------------+--------------+-------------------------+
5 rows in set (1.60 sec)

mysql&gt;


You can get the complete definition by running SHOW CREATE TABLE db.tbl\G from any table from the result of that query.