If this observation is correct:
If two drives (preferably of equal size are set up as a RAID 1 configuration all data is mirrored between the two drives. If an error is detected on one drive the system keeps going with data from the mirrored good sector and the error is corrected on the damaged mirrored sector. The down side is that you only have half the storage capacity of the drives.
It sounds like it would be worthwhile to keep those old hard drives from broken down machines or when a new larger drive is added to your computer and set them up in an enclosure as a RAID system.
I have two spare drives now an 80GB and a 60GB of 2001 vintage -neither drive had caused any problems when I replaced them but I decided to replace them just in case.