Use the inbuilt server backup functionality to back up your server and data to an external 2TB drive. Have a second drive offsite to take into account unexpected scenarios such as fire flood etc. Note that you can not back up more than 2TB data by this method.
RAID is not particularly good for the OS as you can restore it directly from a server backup in the event of failure. It's also not good for data backup (think motherboard failure, fire etc) but good for data availability in the event of drive failure.
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