It should take around a minute and be pain-free - you’ll see a progress display at the bottom right of the Disk Utility window, and can continue working in other apps while it completes. Click the Apply button to repartition, and then Partition in the dialog box that appears.In the Name field at the top right of the window, type something memorable, like El Capitan.If you find the plus button is greyed out and unclickable then you might have to temporarily deactivate FileVault on your hard disk - see below.8GB is a minimum laid down by Apple but you can provide more space, of course, and in any event you may have no choice because the partition will have to be a minimum size laid down by Disk Utility - on our 512GB MacBook this was 33.63GB, for example. Separating the two partitions will be a drag handle, which you should drag so that the new partition at the bottom is given a minimum of 8GB of space - look at the Size field at the right to get an idea.Don’t worry - nothing’s actually changed until you click the Apply button, and you can quit at any time without making changes. This will split the existing Mac OS X partition in half. Underneath the Partition Layout diagram, click the small plus button.Select your hard disk in the list on the left of the program window - the first entry in the list and not the one indented beneath - and then click the Partition tab at the right.Start by opening Disk Utility, which you’ll find in the Utilities folder of the Applications list.However, even they won’t deny that repartitioning is a somewhat risky activity – data is shifted around on a fundamental block level - so you should create a Time Machine backup before following the steps below. This sounds complicated and dangerous but Apple has put a lot of work into making it simple and safe.

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