MySQL General error: 126 Incorrect key file for table '/tmp/#sql_27e6_0.MYI'; try to repair it'
You can remove any /tmp/#sql* files -- they are tmp tables left over after a crash.  Or they are part of something (ALTER, CREATE, etc) that is going on, but stuck waiting for disk space.

It is often a mistake to have a separate filesystem for tmp -- you run into problems like this:  Plenty of space to finish a task, but not in /tmp.  Consider changing mysql's tmpdir setting.