Is there anything in the error log file of the web server? What about syslog?

Does the script work fine if you call it from the same user ID as the webserver uses?

Is selinux or another security enhancement enabled on the webserver?

Is the response from the CGI script rather slow?

Update: You might also consider collecting statistics such as disc usage with tools like collectd or nagios, and then query the collected statistics in your CGI script.