How to Create a disk set in Sierra

1 Create a disk set

Connect to your Mac all of the disks you want to be part of the disk set. For safety, disconnect other disks that won’t be part of the set to ensure you don’t accidentally erase their contents in this process. Choose File > RAID Assistant.

4 Choose disks for the set

Disks and volumes are organised much like they are in the left pane of Disk Utility’s main window, with volumes indented slightly under the disk they are on. Click the checkboxes next to each disk you want to include in the set.

7 Chunk size (striped sets)

With a striped set, chunks of data that make up each file are spread across the disks. If you chose this type for faster transfers, a smaller chunk size is more efficient; one disk can read a small chunk while another looks for the next one.

2 Choose a set type

You’ll be shown three descriptions, corresponding to those on the facing page, 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.

5 Set each disk’s role

Each disk in a mirrored set must be designated as a RAID slice, to contain your data (two minimum), or a spare. Spares are optional but offer some extra protection; they are used to rebuild a set if a slice becomes inaccessible. Click Next.

8 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 for simpler files. With ‘Automatically rebuild’ on, if a disk fails or is removed, the set is rebuilt using a spare; a reconnected disk then becomes a spare.

3 Review 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. To aid in their identification, each one’s connection type and capacity are also listed.

6 Set disk set properties

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

9 Finish creating the set

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