https://forums.zoneminder.com/viewtopic.php?t=24534
& personal message by simon@rustyspoon.co_m

diff -ruN ZoneMinder-1.30.2.orig/web/includes/functions.php ZoneMinder-1.30.2/web/includes/functions.php
--- ZoneMinder-1.30.2.orig/web/includes/functions.php	2017-03-30 20:43:06.000000000 +0200
+++ ZoneMinder-1.30.2/web/includes/functions.php	2017-04-16 21:13:43.841371345 +0200
@@ -1442,7 +1442,7 @@
   return( $load[0] );
 }
 
-function getDiskPercent($path = ZM_DIR_EVENTS) {
+function getDiskPercent($path = ZM_PATH_WEB.'/'.ZM_DIR_EVENTS) {
   $total = disk_total_space($path);
   if ( $total === false ) {
     Error("disk_total_space returned false. Verify the web account user has access to " . $path );