Use an external SSD as your startup disk … (2024)

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Jack-19

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by: Jack-19

If you have an iMac (Late 2015) and earlier or a Mac mini, you'll need a USB-A to USB-C adapter cable, such as the Belkin 3.1 USB-A to USB-C Cable, to connect to this SSD with your Mac.

Although many external SSDs may work as an external SSD startup disk for Mac, some may not have the correct connection or startup disk functionality. If you purchase an external SSD other than what is recommended above, check with the manufacturer to find out if it uses USB 3.1 technology and can be used as a startup disk for a Mac computer.

Connect and erase the external SSD

Connect the external SSD to your Mac using the cable that came with it, or if the cable doesn’t have a connector that fits in your Mac, use an adapter cable.

In Disk Utility on Mac, you can erase and reformat the external SSD to use with your Mac. Erasing a storage device deletes everything on it.

In the Finder, choose Go > Utilities, then double-click the Disk Utility icon. (Disk Utility is located in the /Applications/Utilities folder.) Alternatively, click Launchpad in the Dock, type Disk Utility in the search field, then click the Disk Utility icon.
Choose View> Show All Devices.
In the sidebar, select the external SSD device you want to erase, then click the Erase button.
Click the Scheme pop-up menu, then choose GUID Partition Map. (If the Scheme pop-up menu isn’t available, make sure you selected the storage device you want to erase—it’s at the top of the hierarchy.)
Click the Format pop-up menu, then choose APFS. (If APFS is not shown, choose GUID Partition Map first, then change the format to APFS.)
Enter Macintosh HD as the name.
Click Erase, then click Done.

Install macOS to the external SSD

Apple recommends using the latest (newest) macOS that is compatible with your Mac. macOS updates and upgrades help to maintain your computer's security, stability, and compatibility, and they include the latest features and built-in apps.

Before installing, it’s a good idea toback up your Mac.

To download macOS, see the Apple Support article How to download and install macOS - Apple Support, and look under the "Use the App Store" section to find the version of macOS compatible with your Mac:

macOS Sonoma is compatible with these computers

iMac (Retina 5K, 27-inch, 2019)

iMac (Retina 4K, 21.5-inch, 2019)

macOS Ventura is compatible with these computers

iMac (Retina 5K, 27-inch, 2017)
iMac (Retina 4K, 21.5-inch, 2017)
iMac (21.5-inch, 2017)

macOS Monterey is compatible with these computers

iMac (Retina 5K, 27-inch, Late 2015)
iMac (Retina 4K, 21.5-inch, Late 2015)
iMac (21.5-inch, Late 2015)
Mac mini (Late 2014)

macOS Big Sur is compatible with these computers

iMac (Retina 5K, 27-inch, Mid 2015)
iMac (Retina 5K, 27-inch, Late 2014)
iMac (21.5-inch, Mid 2014)

macOS Catalina is compatible with these computers

iMac (27-inch, Late 2013)
iMac (21.5-inch, Late 2013)
iMac (27-inch, Late 2012)
iMac (21.5-inch, Late 2012)
Mac mini (Late 2012)

When the macOS installer opens and you're asked to choose where to install, click Show All Disks, then select the new external SSD before continuing.

Move your content to the external SSD

You can use Migration Assistant to copy all of your documents, apps, user accounts, and settings to the external SSD from the internal disk or your Time Machine backup.

When your Mac restarts to the external SSD, follow the prompts in Setup Assistant. When asked how you want to transfer your information, select the option to transfer from a Mac, Time Machine backup, or startup disk. Then click Continue. Select your Time Machine backup or internal hard disk, then click Continue. Continue to follow the steps to restore. Large transfers might need several hours to complete.

Finish up

After Migration, change your startup disk to set the external SSD as the default startup disk.

In macOS Ventura or later: Choose Apple menu  > System Settings, click General in the sidebar, then click Startup disk on the right.
In macOS Monterey or earlier: Choose Apple menu  > System Preferences, then click Startup disk. Click the icon of the disk you want to use, then click Restart.

Next, rename your internal disk to a name other than Macintosh HD.

You may need to configure certain settings again, such as Time Machine.

About the internal disk

After setting the external SSD as the startup disk and migrating your data, you can erase the internal disk.

In the Finder, choose Go > Utilities, then double-click the Disk Utility icon. (Disk Utility is located in the /Applications/Utilities folder.) Alternatively, click Launchpad in the Dock, type Disk Utility in the search field, then click the Disk Utility icon.
Choose View> Show All Devices.
In the sidebar, select the internal disk device you want to erase, then click the Erase button.
Click the Format pop-up menu, then choose APFS.
Enter a name.
Click Erase, then click Done.

If you plan to leave the internal disk as-is with macOS on it, exclude the internal disk from Spotlight indexing: Change Spotlight preferences on Mac - Apple Support.