By using RAID. As Eliah Kagan explained in his answer, But keep this in mind:
Ideally you should use two identical hard drives for Raid 1 mirroring. The reason is that both drives will be doing the exact same thing at the exact same time. When you write to one, you’re writing to the other, creating a mirror image of the first. That means both drives will be under the same stress.
Keep in mind too that Raid 1 mirroring will slow your computer because it has to write everything twice. That’s why I recommend the external storage. Put one hard drive in your PC, back it up, and save your data externally.
Back in the days of parallel ATA, it was a no-no to mix drives from different vendors on a RAID controller. Sometimes it would work--more often it wouldn't. With the advent of SATA, mixing vendors is no longer as problematic.
Also, though there are controllers that allow mixing and matching capacities without losing storage space (using advanced parity techniques), these are still relatively rare. Using drives of different sizes usually results in total storage capacity that's only a multiple of the smallest drive. For example, combining 500GB and 320GB drives in RAID 1 would be 320GB * 2, or 640GB instead of 820GB.
In practice, you'll save yourself a lot of time and sidestep possible issues by building your array with identical hard drives--meaning drives of the same make, model, and size. You need at least two drives for RAID 0 and 1, three drives for RAID 5, and four for RAID 0+1, 1+0, and 10.
Hope that this helps in making your decision.