iphone will not connect to iMac - icon flashes on and off in finder and beeps a lot

Iphone will not connect to iMac - icon flashes on and off in finder and beeps a lot

Most link suggest the usb cable is defective i have tried several, i have also tried all the ports on my mac.

iMac and phone are up to date with software and i re-started both.

Any ideas? i want update my music etc