How to delete a disk set using Disk Utility in macOS High Sierra

You can use the Disk Utility in macOS High Sierra to delete a disk set that's no longer needed. When you delete a set, the data recorded on the disks isn't deleted. For example, deleting a mirrored RAID set leaves each disk in the set with a copy of all the RAID data. 

Disk Utility.jpeg

When you delete a concatenated set, all the data is copied to the first disk of the set (as long as the first disk has enough disk space to store the data), providing a single volume with all the data. Before deleting anything, back up any important files. Then:

    •    In Disk Utility, select a disk set that you want to delete in the sidebar.

    •    Click the Delete RAID button.

    •    Click Delete.

    •    To erase all the disks in a disk set:

    •    Back up any important files.

    •    In Disk Utility, select a disk set that you want to erase in the sidebar.

    •    Click the Erase button  in the toolbar.

Remove a disk from a disk set permanently

    You can remove individual members of a mirrored RAID set or concatenated disk set. You can't remove individual members of a striped disk set without first erasing the entire set. You can only remove the last member of a concatenated disk set. 

    •    Back up any important files.

    •    In Disk Utility, select a disk set in the sidebar.

    •    Select the member disk that you want to delete from the set.

    •    Click the Remove button .

    •    Click Remove.

(This how-to is based on my experiences and info on Apple's support pages where the images sometimes come from.)