Installing macOS Big Sur on an external SSD is a great way to upgrade your Mac’s storage capacity and performance. Here’s a step-by-step guide to help you through the process:
Check your Mac’s compatibility: Make sure your Mac is compatible with macOS Big Sur. The minimum requirements are:
MacBook (2015 and later)
MacBook Air (2013 and later)
MacBook Pro (Late 2013 and later)
Mac mini (2014 and later)
iMac (2014 and later)
iMac Pro (2017 and later)
Mac Pro (2013 and later)
Get an external SSD: Purchase an external SSD with enough capacity to hold macOS Big Sur, as well as any additional files and applications you want to store on it. Make sure the SSD is compatible with your Mac’s ports (USB-C, Thunderbolt 3, etc.).
Download Big Sur: Download the macOS Big Sur installer from the Mac App Store. The installer is about 12GB, so make sure you have a stable internet connection and enough space on your Mac’s internal storage.
Connect the external SSD: Connect the external SSD to your Mac using the appropriate cable. Make sure the SSD is properly formatted for macOS (APFS format) and empty.

Launch the Big Sur installer: Open the macOS Big Sur installer from the Applications folder or Launchpad. Follow the on-screen prompts to install macOS Big Sur on the external SSD.
Select the external SSD as the startup disk: After the installation is complete, restart your Mac and hold down the Option key. This will bring up the Startup Manager, which will allow you to select the external SSD as the startup disk. Once selected, your Mac will boot from the external SSD.
Transfer your files and applications: Once you’re booted into macOS Big Sur on the external SSD, you can transfer your files and applications from your Mac’s internal storage to the external SSD. You can do this manually by dragging and dropping files, or by using migration assistant.
Set up your preferences: Once you’ve transferred your files and applications, you can set up your preferences and customize macOS Big Sur to your liking.

Overall, installing macOS Big Sur on an external SSD is a great way to improve your Mac’s performance and storage capacity. Just make sure to follow the steps carefully and take your time to ensure a successful installation.