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I have a Mac Mini with two hard drives inside. The 1st one has an OS installed, and the files I work from. But I wanted to have the second one be used partly for storage, and partly have an OS installed in case the first HD crashes.

I'm imagining that if my main drive fails one day, I will be able to keep working uninterrupted by updating drive 2 with Time Machine.

However in the meanwhile, I'd like to use Drive 2 for storage (while booting from drive 1). I found this article, but it doesn't really say if this scenario would let the user use the rest of the partition for storage if the computer is booted from another drive.

Does this sound like a logical plan, and how would I format and instal the second drive?

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  • The problem is that you can't "partly have an OS installed." It's an all or nothing thing. You can have just the o/s installed. Making the second drive bootable but then if the first drive fails you would have to restore all of your files and apps from the first drive to get yourself up and running. That would work.
    – Steve Chambers
    Commented Sep 2, 2020 at 0:05
  • how big are your drives?
    – Natsfan
    Commented Sep 2, 2020 at 0:07
  • They are 1tb each. Steve, I tried but once there was an OS on drive 2, I couldn’t use it for storage (impossible to copy files to it)
    – MicroMachine
    Commented Sep 2, 2020 at 16:15

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You can partition the 2nd drive leaving ample room for the system software. Then you can install the system onto the 2nd drive or clone drive 1 onto drive 2 and have a backup of everything on drive one. Cloning allows you to use the 2nd drive as a startup drive. The downside to cloning is that you copy the entire contents from drive 1 to drive 2. Installing the system software avoids this problem. But as @Steve Chambers said, if you just install the system and hard drive 1 fails, you need some way to get the data off drive 1. If you clone you have the data from disk 1. But it will become dated the longer you use disk 1. I'd suggest you install just the system on Disk 2 and you should be able to use the other partition for whatever you need.

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Best is nothing exists on a Time Machine volume - it purely is for backup data and needs a little extra space to help copies of changed files and deleted files.

I wouldn’t worry about keeping any OS on the second drive. That limits you unnecessarily and provides little benefit if you have internet recovery.

I would use an inexpensive USB flash drive to have a bootable installer handy to speed up the install of your OS should you have a failure. My estimation is 90% of failures of your primary OS will leave the recovery partition on that drive viable and you won’t need recovery from the internet or a bootable installer. With a little prep you’ll be covered for the remaining 9.9% of failures with only an exceedingly small chance one HDD failure will prevent booting until it’s removed.

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