Trouble migrating Photos library

[flickerflix]

I know that, in theory, one can just drag the photos library to an external disk (just dragging across the Photos package). Photos is running fine on my Mac but it has gotten too large and I need to move it. When I try to do so I get a -43 error and it won't copy. This is on OS X 10.10.5. Any suggestions on how to deal with this?

Thanks

 

[davidmartindale]

Is the Photos app open when trying to copy to the EHDD?