My son is about to get a new 21.5" iMac with the standard 1TB HDD (its on the Fedex truck)! Background is: I would like to eventually get him one or two external SSD for (1) OS & Apps that he can boot from and (2) scratch disk for FCP X and whatever else he is working on and that requries a lot of disk activity (he will probably play around with Blender a bit as well as the video editing but is not into gaming at all). Hope that TB enclosures will be affordable soon! From other threads I gather that a 60GB SSD should be adequate as a scratch drive for video editing. I have a 1TB USB 3.0 HDD that will be used solely as backup using Time Machine. In order to configure the SSD for OS & Apps (when I get if for him) I figure the best bet will be to clone a partition form the internal drive on which all this stuff will be installed from day one; a second partition on the internal drive can then be used for long term storage. Question is: What size partition should I immediately create for the OS & Apps on the internal drive? Should I size it to a comfortable 120GB to match SSD size or is that much more than necessary and if it is much more than needed then I expect a single 120 GB SSD could perhaps serve both (1) and (2) purposes. Appreciate any help on this.