I recently stumbled upon this error when performing a large query on my NAS drive (which also serves as my testing webserver).

This error does mean, that you do not have sufficient memory available in your tmp - directory/partition.
On my NAS Drive there is only 32 Mb available - too less for my monster query.

The solution now is to free up some space, that may work for you, especially if you have some heavy processes running which does fill up your tmp folder.

a) Stop mysql server

# /etc/init.d/mysql stop

b) check filesystem and tmp directories

# df -h

# df -h /tmp

c) remove files from tmp to free up some space

# rm -rf /tmp/*

d) Look into /var/log if you find any uncompressed or large files, get rid of them also

e) last but not least, check and repair your ISAM table

# /var/lib/mysql/myisamchk

Also you can just increase disk space or re-partition to get more space for temporary files

Don't forget to start the mysql server again and recheck your query:

# /etc/init.d/mysql start

Hope this is helpful for someone ;-)