How do I de-RAID a Mac Mini?

Then use the old-fashioned way. Remove at least one drive. Test that you can boot from that drive. Add the second drive preferably renamed. If these are your only drives, then use the second one as the backup. It should contain the same information that is on the first drive. You should be able to select it to also boot the computer. Use a good backup program like Carbon Copy Cloner to do scheduled, incremental backups in order to make this drive an ongoing copy of the startup drive. A mirror without a RAID.

Yes, a Mini can be booted from a flash drive if it contains a version of macOS that can boot that model.

Linux arrays aren't any easier when up against this same issue. Please post an image of your Disk Utility as it shows the drives and as it shows your RAID array. If I can see what is there then I may have a better idea of what is going on.

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RAID support is supported in Disk Utility. I have three of them running here. macOS will run on a RAID but you cannot make a fresh installation of macOS on a RAID. If you have a single drive available then clone or install macOS High Sierra on it. You can then erase the existing array and clone macOS to the array. Restore your data from your backup.

You can also update the system already installed on the RAID. Download the High Sierra installer from the App Store. Work through the installer but don't erase the array. The installer should update your existing system automatically.

What is your machine's model ID: To find the model identifier open System Profiler in the Utilities' folder. It's displayed in the panel on the right. You may also find the complete model information by selecting About This Mac from the Apple menu. Also, you need to do the installation or upgrade to the RAID, not a member disk. The error you received indicates you selected a member of the RAID, not the array that is mounted on the Desktop.

What is currently installed on the machine? Is it the Client or Server? You cannot simpler upgrade the Server.

I would like to stress that a RAID should not be used as a startup device. This is not the purpose of creating RAIDs.

RAID Basics

For basic definitions and discussion of what a RAID is and the different types of RAIDs see RAIDs. Additional discussions plus advantages and disadvantages of RAIDs and different RAID arrays see:

RAID Tutorial;

RAID Array and Server: Hardware and Service Comparison.

Hardware or Software RAID?

RAID Hardware Vs RAID Software - What is your best option?

RAID is a method of combining multiple disk drives into a single entity in order to improve the overall performance and reliability of your system. The different options for combining the disks are referred to as RAID levels. There are several different levels of RAID available depending on the needs of your system. One of the options available to you is whether you should use a Hardware RAID solution or a Software RAID solution.

RAID Hardware is always a disk controller to which you can cable up the disk drives. RAID Software is a set of kernel modules coupled together with management utilities that implement RAID in Software and require no additional hardware.

Pros and cons Software RAID is more flexible than Hardware RAID. Software RAID is also considerably less expensive. On the other hand, a Software RAID system requires more CPU cycles and power to run well than a comparable Hardware RAID System. Also, because Software RAID operates on a partition by partition basis where a number of individual disk partitions are grouped together as opposed to Hardware RAID systems which generally group together entire disk drives, Software RAIDs tend to be slightly more complicated to run. This is because of more available configurations and options. An added benefit to the slightly more expensive Hardware RAID solution is that many Hardware RAID systems incorporate features that are specialized for optimizing the performance of your system.

For more detailed information on the differences between Software RAID and Hardware RAID, you may want to read: Hardware RAID vs. Software RAID: Which Implementation is Best for my Application?

What is below the array's Desktop icon? The Desktop icon and the array's name are together. Installation is to that, not one of the array members. There is no such thing as one of the members being the startup disk. I suggest you try the command: diskutil ar list. You will see the array's Name at the top of the information along with the Device Node. This will list your AppleRAID.

I don't know what the Desktop name of the array is but that is the disk you want for the installer.

In the installer at is starts up you will notice it has selected a destination. Under that is a link to display other eligible disks. Look for "MavericksMirror" and click on it. If you don't see it that means it is not an eligible destination. This will happen if you try to make a fresh installation of macOS on an erased array.

If the array is erased then you cannot install macOS on it. If Macintosh HD is grayed out that may well be because it already is a High Sierra startup volume. I don't yet know what the deal is about your array, but if you have a working system on Macintosh HD that is the Server, then it cannot be upgraded using the Client installer. Eventually, I'm going to suggest that you get a working system on Macintosh HD and upgrade it to High Sierra if needed. Then clone Macintosh HD to your array or use Time Machine to backup Macintosh HD to the array or some other backup software - I recommend getting Carbon Copy Cloner.

If your array is running Sierra and being used to boot the computer, then I suggest you break the RAID and designate one drive as the startup drive. Use the other drive as added storage. There is nothing to be gained by using a RAID as a startup disk.

Do you see anything like my Disk Utility shows? I'm afraid I don't remember if deleting a RAID requires the disks to be unmounted first. What is the specific error you get when trying to delete the array? Note, that you use your RAID as a startup volume, so you cannot delete the array while still booted from it. You will need to boot the computer from another drive.