How do I copy my whole imac harddrive (High Sierra) to external hard drive? Can't drag...

Trying to copy the whole iMac hard drive on my old iMac , running High Sierra, to an external hard drive. Did a clean install on my new IMac and want to slowly transfer files at my convenience. Giving old iMac to a friend. When I try to drag the HD icon to the drive it bounces back.

Help...