You can try the following:
Reset the ownership and permissions on the folders containing the photos as follows:
1 - download and launch BatChmod .
2 - drag the folder containing the photos into the BatChmod window.
3 - set the ownership and privileges settings to that shown in this screenshot:
4 -try dragging the folders to your external HD again.
Note: if the Group that your files belong to are different from staff then you can use it.
Also make sure that the ownership of the external HD has been set to be ignored.