You should rotate those drives away from the computer periodically, so that they're not connected, and -- if possible -- in a completely different location.

I have a 4 disk rotation for my server: "Daily 1", "Daily 2", "Weekly 1" and "Weekly 2". "Daily 1" and "Daily 2" generally stay connected to the server. "Weekly 1" and "Weekly 2" rotate offsite regularly (though not weekly, as it happens -- because I don't actually visit our Cambridge office as frequently as Hugo would like...). This means that I always have a backup that's (reasonably) recent, that's stored somewhere completely different.

Then, every 6-12 months (I'm not rigorous about it), I "retire" one of the disks, so that -- in effect -- "Weekly 1" becomes "2015-07", "2016-02", etc., and I replace it with a new "Weekly 1". This also allows me to smoothly upgrade to larger backup disks as needed.

This is made much easier by the fact that I use BackupAssist (it's not free), which manages the rotation for me.