How to backup/export photos/videos from the new Photos app to an external hard disk?
If you simply want the entire Masters structure copied to a new drive, then 


first find your iPhoto or Photos Library file. Default location for both is ~/Pictures/
Right click the Library file &amp; select Show Package Contents - that will expose the 'file' as the folder it really is.
The Masters folder should be just inside. You can copy that to anywhere.
It is sorted into years, then months. Not all contained folders will actually be populated, if old photos were ever deleted, the file structure remains.


As pointed out in comments, if you need the copied structure to be readable natively on Windows, then you ought to format the target disk as ExFAT.
Windows cannot natively read Mac OS Extended disks &amp; would need 3rd party support.