In the past, the Incorrect key file for table error was caused by an issue with the MySQL database.
Some possible causes are:
1 - /tmp/ folder is not large enough (Linux quota)
2 - table may need to be repaired (run command: REPAIR TABLE `mytable`;)
3 - possibly a bug in MySQL
The following links may provide a possible solution:
http://forums.mysql.com/read.php?21,362974,362974 (esp. first response found here http://forums.mysql.com/read.php?21,362974,363291#msg-363291 )
Hope this helps!