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.

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    RAID is not backup. It's redundancy/high availability. Do not mix the two. If you need backups, continue doing that. If you need HA, get a NAS - but continue with a backup strategy that works for you.
    – parkamark
    Jul 28, 2021 at 12:38
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    You have described how you treat the data, but you hand't described any requirements of your service. How reliable it must be? For how long you can tolerate a downtime? Also, what does not satisfy you in the current setup (do you experience or expect any problems with it)? Any proper advice could only be based on serviice requirements, not on current setup. Jul 28, 2021 at 12:58
  • @parkamark if i continue with one external HDD, should it fail I lose all my backups, the idea of NAS is more drives less likely that I lose my data.
    – S. W. G.
    Jul 28, 2021 at 13:14
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    @S.Redrum Note that having twice as many drives makes a drive failure twice as likely.
    – A C
    Jul 29, 2021 at 3:18
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    @S.Redrum A backup is also intended to protect against accidental deletion of files.
    – glglgl
    Jul 29, 2021 at 15:27

6 Answers 6


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.

  • I dont actually need to store versioning, so I just need to have the latest daily backup for the data, I keep 7 days in FIFO just in case one backup gets corrupted due to backup software (Macrium) used or non hardware causes. I don't need to share anything, was just thinking to replace external HDD with more HDDs (NAS) to lower the risk of disk failure.
    – S. W. G.
    Jul 28, 2021 at 13:19
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    @S.Redrum Excactly thats the reason for nas, reduce the chance of a failure for short-term backups. you may want to chat with me so we dont miss-use this commentary function
    – djdomi
    Jul 28, 2021 at 13:30
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    Ok so storing daily data backup, while maintaining a 7 days line could be take as a short term backup? In short what I need is: preserve the most recent copy of data, preserver a system image that I can restore quickly with all software installed, at this poinr ill just need to update the data from the most recent and I should be fine.
    – S. W. G.
    Jul 28, 2021 at 14:50

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.

  • So more disks is not more safe than just one?
    – S. W. G.
    Jul 28, 2021 at 15:05
  • Yes, more disks are more safe than just one. But you will never be 100% safe. As said elsewhere, RAID is no substitution for backup. I like the fact that you prefer storing backups on RAID in favor of a single external disk and that you do not only rely on this storage for your backups. Jul 28, 2021 at 15:17
  • Ho about a RAID1 with 4 disks? Would that be safer than a RAID10 or 6 or just a dumb idea?
    – S. W. G.
    Jul 28, 2021 at 16:12
  • If you have 4 disks and choose between different RAID levels you have the following advantages/disadvantages: RAID1: Will survive 3 simultaneously crashing disks but only give you capacity as 1 disk, read performance might get 4 x 1 disk performance. RAID10: Might survive 2 simultaneoulsy crashing disks (but might fail depending upon which 2 disks of the 4 that will crash). The capacity will be equal 2 x disk-size and you might get read performance equal to 4 x read performance of 1 disk. RAID6: Will survive if any of two disks fail. The capacity will be equal of 2 x disk-size. Jul 28, 2021 at 16:22
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    Tip: Spread the purchase of drives to multiple retailers and across multiple weeks/months. Don't buy all drives from the same dealer on the same day. That way you can lower the possibility of the other drives failing when rebuilding a RAID 5 or 6 upon one of them failing. It'll still be non-zero, though.
    – Tim
    Jul 29, 2021 at 8:25

Reexamine what point in time you need to recover if the data is lost. Losing a week of work is bad in any organization, but may be tolerable to yours. And test restores, which is the entire point of backups.

Your current backups are good in that you have three copies of the data, and one at a different location. Assuming the weekly backups meet your business continuity goals if both the computer and the external were lost.

A storage array improves reliability by surviving a disk failure. Yes, it could make sense to replace the external with a NAS dedicated to backups. Note that if the primary storage is still one disk, it will fail at the same rate as before and some day require a restore. Which may be suitable for your needs.

Beware that an always-connected file share is not a cold offline backup. Ransomware will encrypt such files if it finds them. Assume your backup user is compromised, what damage can it do to backups? Typical file permission schemes cannot easily implement a user that can write to files, but cannot change them or delete them. (Try doing so with SMB or NFS.) Unplugging the backup NAS from the network when not in use may seem tedious and paranoid, but perhaps limiting access to the daily backups is useful. Confirm that the cloud backups cannot be easily altered or deleted, such as using credentials that do not have delete permissions.

Array type is a choice to make, each RAID flavor you mentioned could be decent. RAID 1 of a pair of drives is easy to do, but only 50% usable space. RAID 6 of a handful of drives will survive 2 failures, very slightly slower due to parity calculations. RAID 10 is striped RAID 1, can survive at least 1 failure, and is quite fast, but 50% usable capacity may be too expensive compared to RAID 6. All of the above are better than the single point of failure that is a single physical drive.

Backup to tape could be a different option. Superior in that cold offline backups are easy, stored tapes are not being read or written to. Further, using a different media type means a backup copy has different wear behavior, and is not vulnerable to specific flaws such as in the primary storage's controller firmware. Challenging in that tape library hardware and software can get expensive.

  • I think both raid 6 and 10 have 50% of total capacity? Tape backup is sequential, it would be really slow to retrieve in case I need a restore. Can ransomware affect data on NAS such as Synology?
    – S. W. G.
    Jul 28, 2021 at 14:48
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    @S.Redrum Yes any device which is beeing connected can be affected, so only a connect and disconnect strategy or software that support suchs kind can incraise this security point but cant in case if they been affected during infection IMHO
    – djdomi
    Jul 28, 2021 at 17:09
  • Ok but I don't need to access my NAS from outside my network, I'll eventually disable the remote sharing of files from devices not on my network. So for instance it should infect my PC to reach the NAS. On my PC I use OSArmor + SysHardener it should prevent or block ransomware from at least phoning home, limiting the impact of such malware. My main concern is hardware failure right now.
    – S. W. G.
    Jul 28, 2021 at 17:46
  • @S.Redrum RAID6 has N-2/N capacity. It's 50% with a 4 disk array with an 8 disk array it's 75%; it's a more fault tolerant variation of RAID5 and intended to let you have bigger array sizes before the risk of a second failure while rebuilding grows unacceptably high. RAID10 OTOH is always 50% because you're mirroring drives. Jul 29, 2021 at 1:41

I had the same problematic as you a couple years ago, and ultimately decided that a NAS and Raid setup wouldn't be what I need.

I decided to disregard the risk of virus and ransomwares, my house burning and most of let's say "less common" problems which may occur, and decided to only focus to protect some medias against disk failure. And at the end, I just decided to add, 2 big HDD, on top of my 2 SSD and 1 HDD in my gaming PC. I copy weekly the content that i want to save on my "1st new" HDD, and i keep the 2nd new HDD unavailable and make a copy on the 2nd HDD when i feel that enough new medias where added to the "1st new HDD", and then on top of that i still had the most important stuff backed up on an external drive, and another pc. (Not that much into cloud solution, and my stuff is located at different places, so I believe it should be fine this way)

And for all of this jungling I use a software made for doing this kind of backups / copies (bvckup2), instead of going all out with a NAS, and all ..

TL:DR : If you "just" want to secure you Datas and are not interested in the others benefits of NAS, making a system only for storing backups can be easier done and used than a whole NAS, with Raid and all, also i hesitated to use Raid 1, but then just decided not to use it, so that i could have one of the 2 drive permanently unavailble, unless i manually re-activate it, so that at least this drive can be partially protected from virus, and those kind of problems.

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    S. Redrum did not ask about your life work history, and is far away from the point of the question. So, either update to an answer that fits the question or please remove it, there are two well-defined Questions. Which he wanted to be answered, and a “NAS” isn't just a big and expensive device. Already any device which is accessible on the network can already be a NAS as it only stands for “network attached Storage” — it usually means a device with at least 2 hard rives, but it doesn't need to have it.
    – djdomi
    Jul 29, 2021 at 14:38
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    @djdomi For his question it's obvious the best Raid setup for his needs is Raid 1, i'm just stating that if he doesn't need, high availability, and "just" care about his Data and only worries about disk failure, he can only add HDD to one of his existing rig, with a decently setup copy mechanism. Then i just explained how i personnaly did it, since i believe both our needs were pretty similar. This is not a general question, so my answer is not a general answer. Your answer on the other hand is pretty useless.
    – user789985
    Jul 30, 2021 at 7:11

If using a small 2-bays NAS your only option is RAID1 (obviously).

If using a 4+ bays NAS for sequential read/write workload (ie: storing files with small or no random access at all), RAID6 should be preferred due to smaller parity overhead (at 5+ bays) and better fault tolerance (any 2 disk can die without data loss). In this case, keep in mind that replacing a disk is going to cause much reduced performance and significant stress on the others.

If random reads/writes are expected to have a significant role (ie: you will be running a database or some VMs off the NAS) or you can't stand the massive performance impact of a parity RAID rebuild, I strongly advise to use RAID10 (parity RAID without BBU caches are very slow in random write patterns).

Bonus point: use a NAS which support snapshots, so you can have point-in-time backup and recovery. And remember that RAID is not a backup, so be sure to keep other copies of your valuable data.



To answer the question posed in your title quickly and succinctly, Raid 1 will always be the most reliable, secure raid level.

If you have ten drives in a raid1 array, NINE OF THEM can fail and you are still online. no other single raid level can survive more than 2 failures

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    And your usable capacity is 10%. Jul 29, 2021 at 17:31
  • @MichaelHampton Where did OP ask about capacity? i saw a vague reference to everything fitting on one drive. If that's the case, fifteen of those drives, in raid1 will be able to store the data in a raid1, or 25 drives, with more reliability Jul 29, 2021 at 17:37
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    This is the real world. There are always other considerations, even if they aren't explicitly stated. Jul 29, 2021 at 17:55
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    Of course, the controller could fail, so you'll want more than one of those... Jul 29, 2021 at 18:29
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    @BillyC. What? You mean if the work PC has a M/B failure that cascades to taking out the PSU and CPU and all the spare parts have been bought by cryptominers? Jul 29, 2021 at 18:55

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