But why do I say that RAID is not backup, then?
- RAID 1-5 can save your data if 1 harddrive fails. If another disk fails before you can replace the bad disk and the data can be mirrord, you have lost all of your data.
- RAID only protects against drive failure. Theft, water damage, virus, accidential deletion etc. will still cause you to lose data.
- RAID does not protect against RAID failure. If the controller, the hardware which manages the disks, fails you will lose all of your data. And trust me: They do fail. Even the very expensive ones.
Note that RAID 6 will allow you to have 2 drives fail before data loss, but: You are still not fully protected.
All in all: RAID will let your server or computer run without interruption if a harddrive fails, but it will not protect you as a backup should do.