Why isn't RAID 1 a backup?
For the reasons listed by other posters. RAID 1
only protects against disk failure, it doesn't protect against accidental deletion of files, corruption of files, power hits to your computer system, theft of the computer system, disasters, etc. etc.
If your data is important, then you really need to protect it against all those risks, and that means you need to have a backup strategy
first, before you worry about RAID. And then you discover that once you have the backup strategy in place you don't really need RAID for data protection - the
real benefit of RAID is to eliminate the downtime that would otherwise be needed to restore your data from a backup in the event of a disk failure.
It turns out that for most home users, it's more important to protect the data against all risks than it is to eliminate downtime - most home users can suffer some downtime without any serious effects, but boy are they pissed off if their precious photos are gone! I see posts all the time from users who are pleading for any way to get their files back, but sadly in a lot of cases it's too late - those files are gone and no amount of money is going to get them back again.
So for most home users RAID is superfluous, at least in terms of data protection (although some may still use it for performance reasons - hopefully they understand what kind of performance it can and can't buy you).
What would be a good backup, an SSD or an enterprise class hard drive?
The three most important aspects of a good backup strategy are:
- more than one copy of the backup. This is typically done by alternating backups to different sets of media - backup to disk 1 this week, disk 2 next week, then back to disk 1 for the third week, etc. This means that you're not SOL if one of your backup disks (or other media) dies.
- more than one generation of backup. This means keeping some backups for a longer period of time - for example you might choose to keep the first backup of a month for a whole year (if you have enough disk space available for that). This protects against accidental deletion or corruption of files that you don't discover right away.
- keep one backup offsite. This protects from risks such as theft or disasters (fire, for example).
If you follow these guidelines, it really doesn't matter what media you use for backup, whether it's DVD/BluRay, tape or hard drive. Hard drives are generally preferred because they're reusable, fast and don't require the kind of manual effort to swap discs that DVD or other optical discs typically do. I personally use "green" hard drives because they're cheap, reliable, and perform plenty fast enough for backups.