My mom runs her own billing service and to protect her server that she runs from she wants to have 2 hard drives with the same info on it. How would you do this?
Yeah, check out what cromewell mentioned. Ist basicly duplicating a copy of your first hard drive. Some other options you could consider, depending on the amount your budjet and amount of data to be backed up, is using a snap server, or tape dirves.
Most mid/high-end motherboards nowadays have RAID1 support but if not you can always buy a basic RAID controller which allows you to run two HDDs in RAID1 mode You can get them as low as $12 although the better models will cost more. A good company to watch for is "Highpoint"