can I transfer photos from camera to portable hard drive then onto iMac???

The easiest way to get the photos onto your iMac is to just plug the camera into the iMac and download them into iPhoto.

see > Mac 101: Connect your camera or camcorder

Then if you want, copy them to the portable hard drive for backup and/or sharing.

A portable HD needs to be connected to a computer in order to mount it and to transfer data to/from it. By itself, it can do nothing.

Either buy spare SD cards (or whatever format your camera uses), or take a laptop instead.