Raid blocks can be a complex system and today I try to explain what are they for.
D1 is when you have the same data, on 2 disks, if 1 fails everything is still on the other disk.
RAID5 is when you have 3 or more disks, and data gets striped. That is, part of the data is on each disk, so no one disk has all the data, the each have part, and so if 1 fails you are fine, if 2/3 failed you would lose all data. Typically RAID5 is considered 1 disk tolerance, so only 1 disk can fail and still be good.
RAID6 is basically raid 5 with 2 disc redundancy
people say RAID is not backup because RAID is not really considered entirely safe from failure. if there is a fire, your raid array saved nothing. if you have a RAID 5 array or 5+ disks, and 2 fail, all data lost. something shorts out in the machine, data could all die. Also, data could all actively corrupt, RAID doesn't protect against data corruption.
A backup can be off-site, it has all the data you want safe, it isn't being actively used so active corruption won't happen
anything bad that happens to your machine won't destroy the data in a backup, but would in just RAID
RAID 0 is if you don't care about the data so much, and just want it to be fast. It is striping without the redundancy. any disk fails and all data lost
A lot of backup software has extra features to ensure data integrity, for safe transfers and storage. also incremental backups are a thing where only changes to backed up data are transfered on updates (saves a lot of time). can also have snapshots of different dates for the data which can save you in some cases
like ransomeware that encrypts all your data and wants you to pay for the key, a snapshot backup of before the ransomware happened and you would be safe.
