How can a Mac split a single backup to two independent, external drives 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?
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.