MacBook Air 2013 : Folder turned into Document WTH?

I have been downloading 4 the last few months into a folder on an external drive connected via USB 3 I am using Mavericks, today I go to download to the folder and I get a message folder not found - when I go into the drives directory I see the folder is now a document of about 650mb in size it has the same name as the folder which was 200gb or so in size. I have not accidentally renamed the folder or changed ext of it myself, I cannot work out how to fix this - I have tried running disk utility and using Diskwarrior also they all state the drive is healthy and the directory structure is fine. Any suggestions would be greatly appreciated this is very annoying and I suspect Mavericks to be the culprit.