From brokenpoet.org wiki

Fixing a table that can't be repaired (says it is "Up To Date" and phpMyAdmin shows it in use) - or just plain regular fixing doesn't work:

Stage 1: Checking your tables

Run

myisamchk *.MYI 

or

myisamchk -e *.MYI 

if you have more time.

Use the -s (silent) option to suppress unnecessary information.

If the mysqld server is stopped, you should use the --update-state option to tell myisamchk to mark the table as “checked.”

You have to repair only those tables for which myisamchk announces an error. For such tables, proceed to Stage 2.

Stage 2: Easy safe repair

First, try

myisamchk -r -q tbl_name (-r -q means “quick recovery mode”). 

This attempts to repair the index file without touching the data file. If the data file contains everything that it should and the delete links point at the correct locations within the data file, this should work, and the table is fixed.