Q I am trying to work out how much free disk space I should keep on two external hard drives, a 2 TB and a 250 GB. Would 10 GB and 1.25 GB respectively be appropriate?
A The correct answer depends on what you intend using the drives for.
A common recommendation is to keep 10% of the total drive space free, which would work out at 200 and 25 GB respectively, but the latter could still prove a bit tight if you were to use it as a startup disk. For that you need ample contiguous free space so that OS X and its many components can keep scratch and other key files, and this depends on how fragmented the free space becomes. For a busy startup drive with applications like Photoshop that keep scratch files you may find it better to try to keep 40-50 GB free, perhaps periodically freshening it up (by cloning off then back onto the drive).
If you will not use the drive as a startup disk, then you can run the free space down to the last 5 GB or so without fear of any adverse consequences.
Updated from the original, which was first published in MacUser volume 27 issue 25, 2011.