I was unable to find a conclusive post on the subject and I discovered how while writing my own question post.
If you create a file on the ReayNAS Duo longer than 255 characters (including path) you will be unable to rename or delete it via CIFS (browsing using Windows Explorer) or the in-built ReadyNAS http file manager.
The http file manager won't even be able to rename the containing directory but windows can.
1) Rename one of the directories in the file's path to be much shorter 2) The file is now deletable - delete it! 3) Rename the directory back. 4) Don't create such long files.
