If you are using SATA drives. Most SATA controllers on modern motherboards
can be setup as a RAID Level 1 system (RAID 1) which takes 2 drives and
pretends they are just one drive. Anything saved to one is duplicated on the
other drive. (MIRROR Mode)
In most cases both drives have to be the same size, as for how you setup a
RAID 1 you probably should list your computer specs. If you bought your
system pre-made, who made it and what's the model number? If you know your
motherboard manufacturer and model list that as well. Someone can probably
walk you through setting it up.