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I have been asked to perform an emergency backup of a device showing signs of HDD failure. I'm being provided with two individual external hard drives to accomplish this. Each drive is independently too small for the job, but together their space is sufficient. I can format them however I want and do anything I want with them.

Can Time Machine perform a single backup that is split across two devices, simply writing to one device until it runs out of space and then writing to the second? If not, can I create a RAID 0 array out of two independent external drives? What would recovery look like if I had to restore such a backup from scratch to a new HDD?

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Time Machine can't partially backup to one drive, then when space runs out, continue to the next drive.

But you can concatenate several drives to look as one and then use Time Machine. As you have said, you can create a RAID drive. This should work with Time Machine. Recovery would look normal. (See here). You can restore from Recovery, Migration Assistant, etc.) Time Machine would detect it as a RAID drive and will mount it as one drive instead of two.

When restoring the entire of the drive I would always suggest booting to Recovery instead of migration assistant.

Just remember, using RAID always significantly increases chances of corruption or failure. RAID 0 is the worst because there are no failsafes: If one drive fails, corrupts, or dies they are both useless.

Your best bet would to purchase a single big enough drive to do the job.

Remember it always better to do it right, especially when it comes to peoples data.

If you are looking to purchase a drive, I would go with LaCie, and definitely not Seagate. 4/5 Seagate drives have failed without being moved within 6 months. None of my LaCie drives have failed.

Note: I am not affiliated with Seagate or LaCie and will not gain in anyway (financially or otherwise) from the purchasing of their products. Additionally, the suggestions above are form personal experience and do not in any way reflect proven published statistics.

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  • Thanks. In this case it's not a permanent solution I'm looking for. I need to perform this backup tomorrow so that if the original HDD dies in two days the people using this system are safe. A better solution will follow in the near future. I appreciate your help!
    – MasterOfNone
    Aug 10, 2018 at 3:34
  • You write that "recovery would look normal". Can you edit your answer to elaborate? This is unclear. Would I simply go into macOS recovery and select Restore From Time Machine Backup, then point to one of the disks from the array? Or would I first install the OS, then boot the OS, then do something like migration assistant, pointing to the RAID 0 array? Would the new hard drive be able to tell that this was a RAID instead of just two disks?
    – MasterOfNone
    Aug 10, 2018 at 3:43
  • @MasterOfNone Will edit. But please check the link.
    – JBis
    Aug 10, 2018 at 3:45
  • @MasterOfNone See edits.
    – JBis
    Aug 10, 2018 at 3:48
  • Thank you. The word RAID didn't appear on the linked page so I wanted to check. I'll mark this as the answer tonight if this goes well today. I'm wondering though, since this only needs to work for a one-time backup, if two independent Time Machine backups, each excluding about half the data, would be safer since a single disk failure would only result in loss of half the data.
    – MasterOfNone
    Aug 10, 2018 at 12:02

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