General Permissions Errors
Note: If you are running in a shared hosting environment, like OIT Web Hosting, then you are not likely to get many if any general permission errors, as each website runs under its own hosting account in these environments. Thus, it is nearly impossible to mess up file and directory ownership in these environments. However, it is worth checking the file and directory permissions, as they are often changeable, and if the owner-write permission is removed from a file or directory then the web server will not be able to modify that file or directory.
If you see any kind of general file or directory permissions errors, the first thing to check is that your sites "sites/default/files" directory and all of its subdirectories are writable by account that your web server runs under. On stand-alone web servers, these directories and their files don't necessarily have to be owned by this account, but the account still has to have permission to access them. Consult with your server administrator if you need help in determining the right user and group ownerships for your website's files and directories.
Temporary Directory Permissions Errors
Sometimes after moving a Drupal site to a new web server, you may see an error message that looks something like this:
The specified file temporary://fileX could not be copied, because the destination directory is not properly configured. This may be caused by a problem with file or directory permissions. More information is available in the system log.
Often this is accompanied by the site looking strange and not having much theming or design. This happens because your site's file system needs permissions to access the Temporary directory. Usually this directory is located at "/tmp", but sometimes the person who built the Drupal site configured it to use a special directory, and then when you copied the site to the new web server, either that special directory was not created or its permissions were not set correctly.
To fix this, try to log into the site as an administrative user and then navigate to the Media section of the Configuration administration page, and go into the File system sub-section. If you have trouble navigating through the site, you might try going directly to that page:
http://your-site-host-name/admin/config/media/file-system
On the File system page, make a note of the path set for Temporary directory and then check to make sure it exists on your server and that it is writable by the account that your web server runs under. Alternatively, you may want to just change the value of Temporary directory to "/tmp" and see if that works. For websites on OIT Web Hosting, the path "/tmp" should always work, and it's a good starting point for many other web servers out there as well. If you still run into problems then you may need to contact your server administrator to find out what temporary directory path you should use with Drupal for that server.