Where else can HDDs be used?

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kurokabau wrote:

Also good to hear that software can read them. I was getting worried that if my readynas failed I'd lose all my data. I wasn't totally sure how the backup drive works in readynas. I currently have 2x 4TB HDDs and one of them is purely a back up I believe (default readynas settings)

It is not really a "backup drive" - and in fact you really should have an independent backup of the NAS data.

Technically what you have now is a RAID-1 array, where the two drives are mirrored.  If a disk fails, then the second one still allows your data to be read/written.  But there are other failure modes (NAS hardware or software, user error, power spikes, etc) that can still result in data loss.

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kurokabau wrote:

 But i don't understand how if I add 2 more 4 TB HDDs, that a single 4TB HDD can back up 3 other 4 TB HDDs.

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One option is to just accept it as "raid magic".  

All the data is available when one disk fails.  None of it is available if two or more disks fail.  So there is no single disk that is backing up the other three.

Technically its a math trick.  Imagine 3 data elements (a, b, c).  You can sum them to p=a+b+c.  Save each of the four on different disks.  If any disk is missing, you can reconstuct the lost data element from the other three.

• a=p-b-c
• b=p-a-c
• c=p-a-b
• p=a+b+c

That's not exactly how its done, but it is the core idea.

As noted above, a backup is still needed to protect against data loss.  RAID keeps the data available when disks fail or when you are expanding the storage - which is very handy.  It provides some protection against data loss too, but not enough.