I recently took delivery of a 27" i7 iMac with 16 GB memory, 3 TB Fusion Drive and the NVIDIA GeForce GTX 780M 4096 MB. Very nifty. But after migrating the 'old' to the new Mac, my external USB 2 TimeMachine drive (volume) started to unmount itself spontaneously. This never, ever happened with the 2009 21" Mac I was using it before. Today I bought a brand new 2 TB external USB 3 drive and designated that as my TimeMachine drive. And lo and behold: it unmounted spontaneously, disrupting the backup. Is this a known problem and is there a solution? Thanks in advance for your efforts!