How to Create a disk set in Sierra

Create a disk set

Connect the disks you want to be part of the disk set to your Mac. For safety, disconnect any other disks, so you don’t accidentally erase their contents. Open Disk Utility and choose File > RAID Assistant.…

Choose a set type

You’ll be shown three descriptions, as we’ve outlined, of the three types of disk set you can create and manage using Disk Utility. Select the type you want to create, then click the Next button.

See available disks

You’ll be presented with a list of all the disks connected to your Mac, and the volumes on them. Your startup disk will be dimmed as it can’t be used. Each one’s connection type and capacity are also listed.

Choose disks

Disks and volumes are organized much as they are in the left pane of Disk Utility’s main window, with volumes indented slightly under the disk they are on. In the list, check each disk you want to include in the set.

Set each disk’s role

Each disk in a mirrored set must be designated as a RAID slice which contains your data (two minimum), or a spare. Spares are optional but offer some extra protection if a disk fails. Click Next.

Set disk properties

You need to assign a name to your disk set, which will be shown in Finder. Of the two choices offered for the set’s formatting, the most common is Mac OS Extended (Journaled) – the one that isn’t case sensitive. Chunk size (striped)

With a striped set, “chunks” of data from each file are spread across the disks. If you choose this for faster transfers, a smaller chunk size is more efficient; one disk can read a chunk while another looks for the next one. Chunk size (mirrored)

In a mirrored set, pick a large chunk size if it’ll store videos or other big files, or a smaller one otherwise. With “Automatically rebuild” on, if a disk fails or is removed, the set is rebuilt using a spare. Finish the set

Click Create, then Done. In Finder, the set acts like a regular disk. To check its status, select it under RAID Sets in Disk Utility’s left pane. You can later add more slices or spares to a mirrored set.