Apologies if this has already been discussed. I am sure it has but this forum seems to move so fast. I am looking to get an external drive for my upcoming iMac. However, I have a few of questions. 1. I presume the drive stays attached to the Mac all the time so that Time Machine can backup when required. If so does the external drive have to be a powered one, as opposed to one powered via a USB or firewire alone? 2. What size (eg 2.5 or 3.5) is the best option and does it matter? 3. There seem to be so many about I am struggling to grasp which are ideal. As this is likley to be on show, I would like a nice looking one?