MySQL Read Speed and Partitioning on Separate Drives
What concerns me fact that fieldC is not the lead column in the PRIMARY KEY.
What would be preferable is to reverse the order to the primary key columns
CREATE TABLE `my_table`  (
 `id` int(10) unsigned NOT NULL,
 `fieldA` char(40) NOT NULL,
 `fieldB` char(40) NOT NULL,
 `fieldC` char(32) DEFAULT NULL,
 -- some other fields
 PRIMARY KEY (`fieldC`,`id`)
 -- some other index
) ENGINE=MyISAM 
PARTITION BY LIST COLUMNS(`fieldC`) (
  partition pNULLorEMPTY VALUES IN(NULL,''),
  partition pValueA VALUES IN('valueA'),
  partition pValueB VALUES IN('valueB'),
  partition pValueC VALUES IN('valueC')
  -- other partitions
);

This may help navigate faster through the partitions. The MySQL Query Optimizer may/may not be smart enough to need the reversal.
If you have other columns in the WHERE clauses such as

WHERE fieldC='...' AND fieldA='...'
WHERE fieldC='...' AND fieldB='...'

you will need matching indexes so quickly search within one partition. Otherwise, you may find yourself doing a full table scan with the partition.
With that in mind, please create the table like this:
CREATE TABLE `my_table`  (
 `id` int(10) unsigned NOT NULL,
 `fieldA` char(40) NOT NULL,
 `fieldB` char(40) NOT NULL,
 `fieldC` char(32) DEFAULT NULL,
 -- some other fields
 PRIMARY KEY (`fieldC`,`id`),
 KEY C_A_Index (`fieldC`,`fieldA`),
 KEY C_B_Index (`fieldC`,`fieldB`)
 -- some other index
) ENGINE=MyISAM 
PARTITION BY LIST COLUMNS(`fieldC`) (
  partition pNULLorEMPTY VALUES IN(NULL,''),
  partition pValueA VALUES IN('valueA'),
  partition pValueB VALUES IN('valueB'),
  partition pValueC VALUES IN('valueC')
  -- other partitions
);

This will then accommodate such WHERE clauses.
Give it a Try !!!
CAVEAT
I noticed your character fields are CHAR and not VARCHAR. That's great.
If any of the character fields are VARCHAR, please read on.
If there are any VARCHAR fields, you need not convert them manually. Simply change the row storage format as follows:
ALTER TABLE mytable ROW_FORMAT=Fixed;

This will make the table's read speed increase 20%-25%. I wrote about this before:

May 10, 2011 : What is the performance impact of using CHAR vs VARCHAR on a fixed-size field?
Aug 12, 2011 : Which DBMS is good for super-fast reads and a simple data structure?
Sep 20, 2011 : Best of MyISAM and InnoDB Jan 03, 2012 : Optimized my.cnf for high-end and busy server