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Question: Time Machine to two external USB drives vs Time Machine to 1 external Raid-1

Recently I purchased an iMac Pro with an internal 2TB SSD and restored my system from an external 1TB HDD Time Machine backup as originally created by my old MBP. For the time being I have continued to use the external HDD for my iMac Time Machine backup, however as this drive is only 1TB it's no longer suitable as a backup target (or at least, the time will soon come where it is not suitable due to lack of space).


My new machine setup is thus:


  1. iMac for day-to-day work, covering everything I do for my business. This is the mission critical device.
  2. MBP for very rare out of office work. I might need to do this once or twice per year.


I've been considering a new backup strategy primarily for my new iMac, and my current thinking is thus:


  1. Replace the external 1TB HDD Time Machine drive with an external RAID-1 device, namely this device G-Technology 12TB G-RAID with Thunderbolt 3 Hard Drive Storage System - Apple (AU) (G-Tech 8TB with hardware RAID, configured to RAID-1), and target Time Machine backups there.
  2. Nightly backups of my internet 2TB drive to Backblaze.


I believe this is effectively the 3-2-1 strategy: 1 internal drive, 1 external drive, 1 off-site backup.


In considering the above I've been researching opinions about RAID-1, and in doing this have seen a lot of opinions stating "RAID isn't backup", "never use RAID for backup" and "deleting files from your drive or being infected with a virus will replicate to the RAID with no roll-back", yet when I consider my strategy above - and bearing in mind for a moment that the RAID drive will only be used for Time Machine backup/restore - I feel like these comments don't apply to my planned configuration. I say this because with RAID-1 I get desirable redundancy protection from drive failures (up to a point, naturally), and as I use Time Machine I inherently benefit from the ability to pick and choose from historical backups. I therefore look at my planned approach and think " I benefit from redundancy, and because I use Time Machine I have multiple versions of each backup, so why can't I use RAID-1 for backup?".

With that said, I have also seen some discussion around using 2 or more external HDDs whereby Time Machine is configured to alternate backups between the drives with each cycle, backing up only what has changed since the last cycle for the specific drive in use. In this setup I could probably buy two 2/3/4TB external HDDs, plug them both in to my iMac, and let Time Machine alternate backups between them.


I am interested in hearing from the experts on this forum their opinions regarding my planned backup strategy, whether RAID-1 as described above makes sense or not (and if not, why not?), and just as interesting to me, what benefits or drawbacks I'm likely to encounter going this route as opposed to simply using two external HDD drives with alternative incremental Time Machine backups. There are two obvious benefits I can already see to using two external HDDs: 1) the cost saving over the external RAID-1 drive (but that is less important to me than reliability and redundancy of backups, not to suggest external drives are inherently unreliable) and 2) if I buy portable external HDDs I can likely use them with the MBP when I'm on the road (this is a very low priority).


Many thanks in advance.

iMac Pro, iOS 11.2.6

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Question: Time Machine to two external USB drives vs Time Machine to 1 external Raid-1