Here is what I need to achieve. I have a work PC with 2xSSD one for O.S. and software, and the second smaller for storing data. Currently I backup my data on an external HDD and on the cloud, I also do O.S. image to recover the full system with software. The data backup on external HDD is daily, the cloud backup is weekly and the O.S. image is monthly.

I wanted to improve this pattern by replacing my external HDD with a NAS, so the first question is: would a NAS used for storing backup files be a good idea or an overkill?

The very second question is: what kind of NAS do I need to store what above? I have read about the various RAID levels, and as of now I'm undecided between 1, 6 or 10 what is the possibility that: a) two disks will go bad at the same time or b) the second disk goes bad before I'm able to replace the first fault?

Another option could be just replacing my current external HDD with a bigger one, that can house more sets of data, but this would not really improve the security the way I had in mind.

The Short Answer is: it depends

The Longer Answer is regarding:

Would a NAS used for storing backup files be a good idea or an overkill?

• In Fact, it is a commonly used practice to do so for short-Term backup
• Long-Term Backups should be saved on Tapes or other medias.

what kind of NAS do I need to store what above?

The Short Answer is: it depends

The Long one is:

It depends on how many drives may you want to have failed at once to still be able to recover.

You may want to read about the Different Raid Level and Information on Wikipedia, as it would explode the Answer dramatically with non-related stuff.

A Commonly used Raid Level for Storing Backups are Raid 5 or Raid 10 - and yes, again it depends on your purposes and needs.

Remind: Raid is NOT a Backup, its just saves you from n-X failed drives.

Where n is your current drives and X the drives where can fail depending on the chosen raid level.

To answer your question about the possibility that two drives fail at the same time: It could happen and it might be more likely than you think. I have seen it happen on a RAID5 system which then got all its data wiped.

Usually the RAID system detects that a drive has gone bad and starts rebuilding on a hot spare or manually replaced bad drive. When this rebuilding takes place all drives in the system will have to work hard and usually all the remaining drives are of the same age, make and model as a drive which has just failed.

On the other hand, I have also seen a raid6 system fail when 6 disks of the same make, model and age decided to give up at the same time. Raid6 would allow 2 disks to give up at the same time, but when 6 of 16 disks gave up all data was lost.