HDD in raid 0 and 1?

In short:

RAID0 = 2 or more disks tied together to make 1 large disk with striping. The controller reads/writes from/to the drives simultaneously so it is in theory x times faster (x=number of drives) than a single drive. Some increase in access times can happen though. If 1 disk fails, whole array is lost!

RAID1 = 2 or more disks that are mirrored to each other (they have the same content). When something is written to disk1 it is synced to disk2 by the controller. If 1 disk fails, operation can continue with the 2nd disk (it is not a backup method). There is no performance increase.

I will echo what GokAy and LMiller7 have to say about RAID not being a backup. As long as a drive is connected to a computer, it is subject to damage from viruses, etc. For a backup to be a true backup, it has to be a copy of the original data and kept separate from the computer except to update the backup.

RAID (Redundant Array of Inexpensive Disks although it up in the air as to whether the I in RAID should be Inexpensive or Independent) was originally more necessary when HDDs were slower and smaller. RAID 0 allowed combining small, pokey HDDs into a single, faster volume. With today's fast SSDs, RAID 0 is no longer really necessary although many geeks like to use it for the impressive benchmarks they can brag about. While, say, doubling the speed of a slow HDD was impressive, doing so with a fast SSD is not so much. Cutting an operation that take a minute down to 30 seconds will be quite noticeable whereas cutting an operation that takes 1 second down to half a second will be barely noticeable. Add to that the time it takes to initialize the RAID during boot up and the gains diminish or even disappear. One serious disadvantage of RAID 0 is, if one drive in the array fails, the data on all the drives in the array will be lost. The only proper way to backup the volume of RAID 0 array is with a volume of equal or greater size. RAID 0 is really past its time but many people still cling to it.

RAID 1 is also called mirroring since a mirror copy of the primary disk is made on another disk. For a small business that cannot tolerate downtime from a failed HDD, RAID 1 would be a minimal solution to ensure continuous operation until the failed HDD can be replaced. However, since the disks in any RAID have to be continuously connected to the computer, RAID 1 cannot be considered a true backup because those drives can be damaged by malware, user error, etc. Again, data is not safe unless a copy of it is kept on a drive that gets connected to the computer only while updating the backup.

For most people, RAID 0 and 1 (as well as other forms of RAID) are unnecessary and can be dangerous if people have unrealistic expectations of them.