I would recommend you start with two backups (no RAID, use the drives for backups) on two separate drives located in a NAS or another computer using something like acrnis true image to do automated scheduled full and incremental backups.
If you think about the possible things you need a back up or RAID for, you begin to realize why you need more than just a RAID or a single backup drive.
You can have
controller and computer driven errors,
software errors and
A backup on a RAID is a good idea but in perspective a RAID is only for HDD errors because it does not address any of the other errors; if your controller goes bad, it will write bad data to your RAID and you need a backup to save you. But switch it around and have a controller go bad. Even if it writes bad data to the backup, with a current backup and a second backup (1 week to 1 month old) you are saved no matter what! This means it is more important to have a good backup that covers ALL the errors, sacrificing RAID if you need the extra space, before you use a RAID. So first, make sure you have enough room for three copies minimum on three different mediums. Here are the three copies.
- The copy you use, everyone using a computer has this copy. This copy is not a backup, it's the files you use, but it is one of the three. This is on your computers hard drive
- The Backup most people think of; the one where you do a full backup then incremental backups for a while. This backup is outdated the length of time between incrementals. This is on the first backup drive
- The backup most people forget about. Think about it, you do a full back up. Then you build on that full backup for some time, a week, a month, a year, what ever you feel comfortable with. IT guys many times do a week. But you hit a point where you need to do a full backup AGAIN so you can start building again on a fresh full backup, and you need to keep the last full backup during it so you are not without if something goes wrong during the backup process. This is on the second backup drive.
This is how it works. A general IT solution is
Full backup sunday the 1st (backup 1), and incremental backup daily.
Then, another full backup (backup 2) on sunday the 7th on another drive leaving the last week's backup (backup 1) intact.
Do incrementals on the new full backup (backup 2) throughout the week.
Overwrite backup 1 on sunday the 14th leaving the recent one from the 7th (backup 2) intact. Then you rotate backup 1 and 2 ALWAYS having a backup, even when you overwrite a backup.
So point is, you need enough space for 2 full backups on two separate mediums first before RAID. Then, after you have a minimum backup scheme, I would do a RAID for my active computer, NOT for the backups. Here's why.
Think of a RAID as a single drive that gives you a chance to fix it while it is "BAD" (i.e. a drive fails). But it does not help with hardware, software, or user errors which are also highly probalby if you ad the chance of all three together. Two backups of different age on two drives drives in a non-RAID does help with ALL the errors.
But a RAID will do great for the files you use actively every day incase of hard drive failure; that is you want to use the RAID for the drives you store things on everyday so you have a level of protection in between backups. Move your My Documents there, store your pictures and emails and such there. The more active the folders are, the more likely something will get missed inbetween backups, an the more they should be located on a RAID.
Don't forget, the more diverse the storage media, the better (external drives are not good, internal drives are good, tape is better still, cloud (like amazon S3, not dropbox) is better than tape and internal drives for reliability, but not for speed)
The farther backups are from the original data, the better. A NAS or other computer is good, a drive or tape written to then stored off site is better, cloud (amazon type) is best.
A complete scenario includes different reliable mediums at diverse distances. Start with user files on internal RAID. Then, backup (incremental style) to internal drive (or another RAID) daily or more often. Then copy that back up out at the end of the month to another drive, tape, or the cloud freeing up your internal drive for the next month of backups. A scenario like this is not that expensive and is near enterprise level reliability only hindered by consumer grade hardware, software, and consumer user. But START WITH BACKUPS, then RAID.