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I have many TB of video & I’m trying to decide whether to get a small number of big backup drives vs a larger number of smaller drives. Cost per GB is cheaper with the big ones, but I worry having so many eggs in one basket.

Imagine 4TB of data, uniformly distributed in value (e.g. all files are of equal value). Is it more reliable to keep the data on one 4TB drive, or spread it evenly over two 2T drives?

At first blush, it seems the two scenarios are equivalent. With two drives, over any given time interval the probability of experiencing a drive failure is doubled, but the consequences are halved. Thus, the long term expectation of the consequence is the same.

Nevertheless I still worry about piling up so much on one drive.

Remember… this is a question about reliability. Are two half-size drives equal/better/worse than one full-size? My gut wants to say more smaller is better, the math suggests they're the same. Am I missing anything?

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    The correct answer is (a) use RAID 1 or better, and (b) make backups.
    – Chris Davies
    Dec 30, 2019 at 16:43
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    Hard drives do fail: backblaze.com/blog/hard-drive-stats-q2-2019 I have data on DVD, flash drives & 3 computers one in different state. I do not yet need TB sized back up. My limited Music needs, I still also have original DVDs. ubuntuforums.org/showthread.php?t=2368992 My backups are not over Internet, so not encrypted. RAID is not backup. smallnetbuilder.com/nas/nas-features/…
    – oldfred
    Dec 30, 2019 at 16:43
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    To build off of what user roaima is saying, if you have 4 TB of data that you absolutely never want to lose you need at least 3 4 TB's worth of disks. 1 4 TB being the original copy on you computer or network storage location. 1 4 TB being a backup copy either also attached to the previous computer/storage solution or kept on an external drive. Note that RAID does not count here. And lastly, 1 4 TB copy kept off-site, either "in the cloud", in a safety deposit box, at a friend's house, etc. Obviously, this has costs so you need to weigh those against how important this data is.
    – kemotep
    Dec 30, 2019 at 16:51
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    @roaima I do not mean to suggest that RAID is a poor choice ever, just that in the context of backups RAID does not count in my opinion. Any RAID option that has redundancy built in works but if that is the only "copy" of the data than it is no better than having just 1 disk with the data. I was suggesting to follow the 3-2-1 policy of at least 3 copies, at least 2 mediums, and at least 1 copy off site. When it comes to important data more copies, and a reliable and tested recovery system, the better. RAID with redundancy is great for handling disk wear-and-tear by allowing "hot swaps".
    – kemotep
    Dec 30, 2019 at 17:45
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    4TB disks are pretty much the manufacturing sweet spot for non-enterprise drives at the moment. I'd recommend WD "Red" if it's for a NAS.
    – Chris Davies
    Dec 30, 2019 at 17:56

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Basically it comes down to the question of how valuable and replaceable your data is.

  • If your data is time-consuming to replace, then consider 2x 4TB disks in a RAID 1 configuration.
  • If your data is irreplaceable, ensure you not only have RAID 1 but also have backups. Ideally off-site backups.

Your 1x 4TB suggestion provides no protection for your data when the disk dies. Your 2x 2TB suggestion is broadly speaking equivalent to RAID 0, except that for true RAID 0 the typical time for failure in such situations is halved (i.e. twice as likely) at the expense of faster read/write access.

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