Rita, many years ago I gave up partitioning hard drives because I found no matter what partitioning scheme I came up with it was almost inevitable I would find space I desperately needed on one partition (volume) was on the other volume. While Disk Utility, and some third party utilities can enlarge a partition it is a process fraught with the risk of losing everything and not one I would want to attempt, particularly where a Time Machine volume is involved. That said, since you are determined on your course of partitioning your external drive, I recommend using the minimum space necessary to contain your clone plus the inevitable growth in OS X, applications, data file plus the space needed for swap drives, and the myriad other invisible temporary files created and used by your applications. That leaves the maximum space possible available for your Time Machine backup.
You have talked about having two "images" one Lion and the other Mountain Lion. That is not as straightforward as it may seem and presents major complications with applications and more importantly, data because of backward compatibility issues from Mountain Lion that I don't think you want to take on so I am assuming a Mountain Lion clone only.
You have previously stated…
I use very little of my Hard Drive so it shouldn't be a problem. I have a 500 GB HD and a 1 TB external drive (when it comes). I am presently using 16 GB. I will probably download a utility now and then, but that will probably be it. I barely used any of my iBook G4 drive and it was only 60 GB.
With that in mind, I would recommend a partition scheme of:
- Mountain Lion Clone volume — 50 GB
- Time Machine Volume — 950 GB
These numbers will be slightly different because of space required for the GUID partition data but this should be close enough.
