I currently have a 1tb iMac and I'm looking into getting an ext thunderbolt connected hdd. Before I buy, I'm hoping someone can clarify for me some questions. If my iMac is 1tb, do I buy 1tb hdd or maybe 2tb or 3tb? The reason why I ask is, let's I currently have 500gb of data on my iMac. Of course, the first back will 500gb, half, if I were to buy a 1tb ext hdd. So my question is, on the second back up assuming no changes and still at 500gb, does Time Machine write another 500gb to the ext hdd so now a total 1tb of back up data? Then, if (assuming no changes), it's time for another back up and it writes another 500gb, will it over write the first 500gb? If it doesn't work like this, can someone explain how and what data TM actually writes so I know if I can stick with 1tb or if I should get 2 or maybe 3 tb ext hdd? Also, if I do back up to TM and let's say i sell me computer, am I able to use the TM back up to migrate all of my files (word files, photos, videos, excel files, etc) to the new computer by connecting the ext hdd and "restoring" the new computer with the TM back up information?