If you work with massive files—such as 4K video footage or large raw photo libraries—you can quickly run out of space on external hard drives. Instead of constantly shifting files across multiple small drives, you can use a powerful feature built into macOS to combine two or more physical hard drives into a single, massive logical drive known as a Spanned Volume (specifically, a concatenated disk set, or RAID 0 array).
When you combine drives this way, macOS treats them as one giant hard drive. When the first drive fills up, data seamlessly spills over onto the second drive without you having to manage any file locations.
Understanding the Risks (RAID 0)
Before proceeding, you must understand that combining drives in this manner offers zero redundancy. If you combine two 2TB drives into a single 4TB volume, and one of those physical drives suffers a mechanical failure, you will lose the data across the entire 4TB volume. This setup should only be used in conjunction with a robust, separate backup system (like Time Machine or Backblaze).
How to Combine Your Drives in Disk Utility
You can create this spanned array entirely through software; no special hardware or RAID enclosures are required.
- Plug all the external hard drives you wish to combine into your Mac. Ensure they are powered on and recognized by the system.
Warning: This process will completely format and wipe all data currently on these drives. Ensure they are empty or backed up. - Open Disk Utility. (You can find it by pressing Command + Space and searching for “Disk Utility”).
- In the top menu bar (next to the Apple logo), click on File > RAID Assistant…
- A new setup window will appear. Select Concatenated Disk Set (JBOD) and click Next. (Do not select Striped or Mirrored for a simple spanned volume).
- You will see a list of all drives connected to your Mac. Check the boxes next to the specific drives you want to combine into the new volume. Click Next.
- Configure your new giant drive:
- RAID Name: Give the volume a name (e.g., “Massive Storage”). This is the name that will appear on your desktop.
- Format: Choose APFS (if you are only using Macs running modern macOS) or Mac OS Extended (Journaled) for older systems.
- Chunk Size: Leave this at the default setting (usually 32K or 64K).
- Click Next, and then click Create.
Disk Utility will now unmount the individual drives, reformat them, and bind them together. Depending on the size and speed of the drives, this process may take a few minutes.
Once complete, a single icon will appear on your desktop labelled with your new RAID Name, representing the combined total storage capacity of all the disks you connected. You can now use it exactly like a normal hard drive.