I am trying to build a backup plan for my hard-drive, and I wonder whether I need to backup all the ".svn" subfolders?
Note: the repository is not on my computer - my computer contains only a working copy.
You don't need to backup these .svn folders. Besides of that, you don't really need to backup your working copies since you can always checkout them from the server again.
However, if your working copies are not large -- there is nothing against backing them up as all other data on your system.