iMac doesn't recognise phone attached via usb

I have an iMac and originally I had an iPhone. For certain reasons I have had to change to a Samsung Galaxy S3.

Originally when I plugged my Samsung into the iMac via usb cable it connected without any issues, however recently when I connect it the iMac does not recognise that there is a device attached. However, the phone still charges.

I have tried this on another computer (a pc) and it is recognised, I was just wondering if anyone knows why the iMac has trouble finding the device when it had no issues originally.