.mov file in external HD, greyed out and won't open: "this item is used by Mac OS X"
This is a common issue which happens when the file's type and creator attributes are modified by Finder when its writing files, if these attributes aren't restored back to the original values by Finder at the end of its write action, those files won't be accessible to the user. 

Running this command will remove the creator and type attributes set by Finder,

SetFile -c "" -t "" path/to/file.mov


Note: If the above command is not found, try installing Xcode command line tool and try the above command again,

xcode-select --install