Gambler wrote...
I do not keep old anime in my hard disk, I burn them to DVD. :D
Jokes aside, I believe it is difficult to come up with an actual answer. Everyone has their own favourite series they would like to keep, but would you rather delete one you have already finished watching or would you rather delete another that is left unwatched? In addition, with new titles appearing every season, one of those series in the priority list will eventually get replaced.
yea, i just burn em to dvd for later use, i don't have any series on my HD that isn't from this season with the exception of specials and 1 series i haven't seen yet from few seasons ago, also once i burnt a dvd and any kind of specials, ova etc some out they stay on the HD cuz the actually series isn't there anymore, so i wait till i have enough to make one big cd/dvd of specials