Re: Is it ok to backup data like this?
The problem with using those other two drives as backups is that you're still decreasing their mean time to failure by running them when they're not needed (unless you don't have them powered constantly).
Ideally, a raid 5 would be the best fault tolerant and redundant option for the backups. Placing this in another device other than your primary file-server is best. Depending on your security concerns and budget, off-site backups could be good or bad. Bandwidth depending on your volume of data could be very expensive.
Hope this helps.
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