How to Set Up an External SSD as Your Apple Startup Disk

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Learn How to Use an External SSD as a Startup Disk for Your iMac or Mac Mini

If you’re looking to boost the performance of your iMac or Mac Mini, using an external solid-state drive (SSD) as a startup disk can make a big difference. Using an external SSD will improve overall system responsiveness, speed up app launches, and enhance your Mac’s performance during startup.

What You’ll Need

A compatible Mac that has USB 3 ports and is running macOS High Sierra (10.13) or later.
An external SSD with sufficient space for your files, applications, and media. Aim for SSDs that utilize USB 3.1.
If your Mac is an older model (iMac from Late 2015 or earlier or Mac Mini), you may need a USB-A to USB-C adapter cable.

Compatible Mac Models

iMac (Retina 5K, 27-inch, 2019)
iMac (Retina 4K, 21.5-inch, 2019)
iMac (Retina 5K, 27-inch, 2017)
Mac Mini (Late 2014 and Late 2012)

Recommended SSD
For a reliable option, consider the LaCie Rugged Mini SSD available at the Apple Store.

Step 1: Connect the External SSD
Plug your external SSD into your Mac using the provided cable. If needed, use an adapter to connect it.

Step 2: Erase and Reformat the SSD
Before using the SSD, you’ll need to set it up using Disk Utility. This step clears any existing data and ensures it works properly with your Mac.

Open Finder and select Go > Utilities. Double-click on Disk Utility.
Choose View > Show All Devices.
Select the external SSD in the sidebar and click the Erase button.
Choose GUID Partition Map as the scheme.
Select APFS as the format.
Name it "Macintosh HD" (you can change this later if you wish).
Click Erase, and then Done when finished.

Step 3: Update macOS
Make sure you’re using the latest version of macOS that’s compatible with your model. This helps keep your Mac secure and stable. For instructions on how to update, check Apple’s guide.

Step 4: Download macOS
Get the latest macOS version compatible with your Mac. You can find a detailed guide on where to download from Apple’s support page.

Step 5: Install macOS on the External SSD
When the installer opens, choose the external SSD as the installation disk by clicking Show All Disks, and then select the SSD. Follow the prompts to complete the installation.

Step 6: Migrate Your Data
After installing macOS, use Migration Assistant to transfer your apps, files, and settings from your internal drive or Time Machine backup.

When your Mac restarts, follow the prompts in Setup Assistant.
Choose the option to transfer from another Mac, Time Machine, or startup disk and click Continue.
Select your internal drive or Time Machine backup to start copying your files.

Step 7: Set the External SSD as Your Startup Disk
After the migration:

For macOS Ventura or later: Go to the Apple menu  > System Settings, click General, then Startup Disk.
For macOS Monterey or earlier: Go to the Apple menu  > System Preferences, then click Startup Disk.

Select the external SSD and restart your Mac to complete the setup.

Step 8: Optional – Erase the Internal Disk
If you decide to keep your internal disk as a backup, you can skip this step. If not, to erase it:

Return to Disk Utility.
Choose View > Show All Devices.
Select the internal disk and click Erase.
Format it as APFS and give it a new name.
Click Erase, then Done.

Final Touches
After everything is set up, remember to rename your internal disk to avoid confusion and adjust any settings like Time Machine back to normal.

By following these steps, you’ll significantly enhance your Mac’s performance with an external SSD as your primary startup disk. It’s a simple yet effective way to breathe new life into your computer!