How do you fix a MySQL "Incorrect key file" error when you can't repair the table?

You must change the location of MySQL's temporary folder which is '/tmp' in most cases to a location with a bigger disk space. Change it in MySQL's config file.

Basically your server is running out of disk space where /tmp is located.

You'll need to run this command from the MySQL prompt:

REPAIR TABLE tbl_name USE_FRM;

From MySQL's documentation on the Repair command:

The USE_FRM option is available for use if the .MYI index file is missing or if its header is corrupted. This option tells MySQL not to trust the information in the .MYI file header and to re-create it using information from the .frm file. This kind of repair cannot be done with myisamchk.

Your query is generating a result set so large that it needs to build a temporary table either to hold some of the results or some intermediate product used in generating the result.

The temporary table is being generated in /var/tmp. This temporary table would appear to have been corrupted. Perhaps the device the temporary table was being built on ran out of space. However, usually this would normally result in an "out of space" error. Perhaps something else running on your machine has clobbered the temporary table.

Try reworking your query to use less space, or try reconfiguring your database so that a larger or safer partition is used for temporary tables.

MySQL Manual - B.5.4.4. Where MySQL Stores Temporary Files