iPhone backup problems

[hakr100]

When I try to backup my iPhone 5s via itunes to my iMac or icloud, it bombs out on me and I end up with the Beachball of Death.

But backup works properly on my macbook air.

So, apparently, there is something amiss on the iMac. But I have no idea what it might be.

Any suggestions of where to start to resolve this issue?

Thanks!