Raid

RAID (Redundant Array of Inexpensive Drives) 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.
It won't protects you against dropping the unit (kids, cats, dogs, drunk), water, fire, theft, etc.

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 stupidities are stored redundant.

Mean Time Between Failure for a HD is 500,000 hours.
What is yours?
Make a backup before you say to yourself "I should have made a backup"!

I backup all the PCs to a NAS
The NAS is synchronized once a week to a NAS at my sister’s house over the internet.
This done using Rsync, very frugal on bandwidth as only blocks with changed contents are transferred.
Basically Rsync is a kind of editor….

You can do both.
Rsync requires the 2 NAS to be in a network, might be local (LAN), might be global (WAN).
What I have done is a Rsync in the LAN (indeed for performance sake and to inspect the results) then moved one to my sister's house.
But you can also do a copy from NAS1 to NAS2 using Explorer, FTP, USB, etc. any other possible mode of data transfer local and then switch to Rsync for a sync over the internet.