.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

First, a simple workaround:

Drag and drop the greyed movie file to QuickTime Player, or VLC etc. For some reason it works if you do it that way. (at least on Yosemite)

Partial solution:

If you need to copy affected files, or you simply want to "ungrey" them and work with them as usual (double click to open etc) then delete the FinderInfo metadata from the file with this command:

xattr -d com.apple.FinderInfo file.mov

You can see the metadata with:

ls -l@

Unfortunately, for me the FinderInfo metadata (and consequently grey files) reappear every time I remount the ntfs disk on which I copied files using native OS X NTFS write support enabled through fstab. So until someone finds a lasting solution the workaround happens to be quite useful.

It's interesting that files residing in the same directory but copied from a windows machine never get metadata.

You won't be able to run Repair Disk on a Windows NTFS filesystem, and NTFS volumes cannot be indexed by Spotlight unless Paragon NTFS or Tuxera NTFS have been installed because Spotlight needs to write its index files to the volume.

I suggest you move these movie files to an HFS+ volume, either to the local HD, or to an external HD formatted with HFS+. You may be able to play the movies with Paragon NTFS or Tuxera NTFS installed, but it may be easier just to transfer the file to a Mac filesystem. If you want to edit these files, you will need to transfer them.

I don't have enough reputation to comment, but adding to the great answers above, I navigated to the drive and used a command listed above to clear out the finder info that was added:

cd /Volumes/[drive name]
xattr -d com.apple.FinderInfo *

and then had to tell spotlight not to index that drive (NTFS formatted drive).

This was a good resource — http://osxdaily.com/2012/01/24/stop-spotlight-from-indexing-time-machine-backup-volumes-external-drives/