Large file Downloads can fail when using Drupal's private file setting due to the script timing out.
Apply the following patch to file.inc in Drupal 5.
Index: filemanager.module =================================================================== RCS file: /cvs/drupal-contrib/contributions/modules/filemanager/filemanager.module,v retrieving revision 1.18 diff -u -r1.18 filemanager.module --- modules/filemanager/filemanager.module 14 Sep 2006 06:06:30 -0000 1.18 +++ modules/filemanager/filemanager.module 6 Nov 2006 22:00:16 -0000 @@ -1040,8 +1040,11 @@ // Transfer file in 1024 byte chunks to save memory usage. if ($fd = fopen($source, 'rb')) { + set_time_limit(0); while (!feof($fd)) { print fread($fd, 1024); + ob_flush(); + flush(); } fclose($fd); }