If there is an SQL problem, the moodle should report the error it got from SQL server.
Attached is a simple patch which does that.

It has already found two errors which were not due to suggested "maybe the disk is full" error:

1) The SQL error was:Lock wait timeout exceeded; try restarting transaction
fixed by "innodb_lock_wait_timeout = 600" in mysql configuration

2) The SQL error was:MySQL server has gone away
this was due to out-of-memory condition which killed mysql server