Confusion over Seagate versus new iPhoto backup
New iPhoto version has been installed. My MacBook Air is running out of start up disc space and wanting to clear some space I copied the photos onto the Seagate slim back up drive. I closed then opened the seagate to check photos were indeed copied. I then went into iPhoto and deleted permanently a whole bunch of photos. When I next opened the seagate, it also showed me that those photos are gone! How can what I delete on my mac also be deleted on the seagate?
MacBook Air (11-inch Mid 2012), OS X Yosemite (10.10.3)