RAID-1 is what you want, but the issue is that if you inadvertantly delete something or change a system setting the change will be automatically copied by the raid onto the backup drive.
Raid-0 has always sounded to me like a disaster waiting to happen
as the scheme is used to increase speed of the "Array" and files are distributed in pieces on ALL the discs in the array.
What it sounds like to me is that the previous owner of your computer
OR the joker who set it up did so in RAID-0
IF you lose ANY drive in a Raid-0 array you lose it all.
There is a good explanation of RAID on Wikipedia:
If you are mostly worried about preserving your data (media files, etc)
Raid really isn't necissary except as a convenience to save you from
manually copying everything to a backup storage drive.
where a Raid-1 shines is for preserving your system setup
because if you have two drives in a raid-1 array and one of
them dies your system still works.
my personal next step is setting up THREE 500gb drives in TWO DRIVE Raid-1
array and swapping one of the three drives out of the computer on a weekly basis
(if a drive isn't spinning it isn't wearing) another benefit of this is after the second
rotation none of the drives will be exactly the same age so having two fail at the
same time is even more unlikely and even if it does happen worst case you have
a week old backup onhand.
But even so my "Boot setup" and system files will be installed into
a relatively small partition on each drive, mainly to limit the inevetible
fragmentation that occours on a boot drive.
I only mention my own plans to give you some ideas...