“for their–there–they’re sake .
(Homework: Which one should I have used?).” May I suggest their ‘sakes,’ not ‘sake’ i.e., unless they all have the same ‘sake,’ which is not what I think you meant.

The second example, shown in ‘Definitions,’ would be correct provided that we know that more than one person owns the hard drive. I’m too old to remember the rule verbatim, but it has something to do with agreement of number, I believe.

These tutorials are so good that I feel at liberty to point out minutiae without in any way detracting from their validity. They are a wonderful way to pull a mind that has gone sloppy back to the fundamentals. In any case, I thank you for your attempt to do something about these frequent and very annoying errors.