If I have two disk drives c and d and I image the c drive to d and leave it for a day; it is a form of backup, since I can revert to the day I mirrored it. But if I mirror it as RAID1, it is not a backup since I'm making it redundant?
What if my sole purpose is just to make the system alive and not reverting to any previous date whatever. Isn't that a form of backup?
No, it is not. For example, if an important system file is corrupted or accidentally removed, your system will be unusable.
No. They are different !
When you have a backup (or snapshot), you know that you can revert back to a point in time. It's your safety for the human factor (tampered files).
On the other hand, redundancy is your safety for the machine factor (fires, disaster, etc).
For example, you have a file and you overwrite it with garbages. If you have a backup, you can move back to the "previous state" by replacing the current file with the backup-ed file. With redundancy, the latest file (the garbages) will be propagated to more than one location (but the garbages will be safe even if your main location burnt into ashes).
Under circumstances, you can regard your backup as skewed redundancy. For example, if you copy your data from a primary location into a secondary every 5 minutes your have a "backup". If you do it every 5 seconds "redundant" (depends on your time constrains). Without being completely true, you can claim that you have redundancy when you write to more than one location in a single write transaction. On the other hand, backup requires first to write your data into a primary location, and then on the backgroud, the data will be copied to the secondary
yes it is a type of backup but other parts of your hardware may fail and still not have an operational PC. It is a form of backup for the data from your PC.