When trying to upgrade WordPress on one of my sites I kept on getting the following error:
“The update cannot be installed because we will be unable to copy some files. This is usually due to inconsistent file permissions.: wp-admin/includes/update-core.php”
I found the answer on the following page: http://aaronjholbrook.com/wordpress-permissions-update-error-resolved/
To fix the problem I ran the following two commands:
sudo find /path/to/site/ -type f -exec chmod 664 {} \;
sudo find /path/to/site/ -type d -exec chmod 775 {} \;
In addition, the user and group must be set what Apache is running under. On my Ubuntu servers that is www-data:www-data and the command, run from the root of the website (usually the wordpress directory) is:
sudo chown -R www-data:www-data {.,}*