I rarely synchronise the iPhone with my Mac, since unlike earlier phones there is so much you can do over the air these days.
Syncing it last night I was slightly confused over why it wasn’t syncing. I unplugged and replugged the iPhone into the Mac. No luck.
A quick search on Google found that sometimes problems arise if you have an USB drive plugged into your Mac. I knew I had plugged in a USB 32GB stick into the Mac, I unplugged it and removed it, and then the iPhone was able to sync no problem.