RAID

Redundant Array of Inexpensive Drives (as named by the inventor) or Redundant Array of Independent Disks (a name which later developed within the computing industry) — is a technology that employs the simultaneous use of two or more hard disk drives to achieve greater levels of performance, reliability, and/or larger data volume sizes.

RAID comes in many flavors but the consumer in general uses raid 1, mirroring.
One disk is an exact copy of the other so if one fails you still have all of your data on the other drive.
This is an excellent way to protect you against loosing your music collection due to hard disk failure but it won't protects you against dropping the unit (kids, cats, dogs, drunk), water, fire, theft.

It won't protect you against user errors like deleting the wrong files or applying the wrong tags. Sounds logical but I have the feeling that a lot of people think they are save because they have a RAID system.

As far as user errors are concerned, the difference between RAID and a single HD is that with RAID all your user errors are stored redundant.
So do make a backup.

An alternative for RAID and an excellent backup strategy is to use remote replication.

You have two units, one in the basement, one in your study or even better in the home of a relative and you synchronize the units over the internet.

Tools like Rsync can synchronize in a large collection is a very efficient way.

 

USB-Hub and thumbdrives used as a RAID 0 (striping)