To setup a clean External USB bootable drive your going to need 1 x USB drive (ideally a SSD with the option for USB and USB-C connection) a spare Mac to use as your clean boot machine and a clone software programme to go the clone once you have setup the clean boot machine on the spare Mac.
To boot a Mac from an external drive, follow the steps below based on whether you’re using an Intel-based Mac or an Apple Silicon Mac (M1/M2/M3).
🔁 Step-by-Step: Boot a Mac from an External Drive
✅ 1. Prepare the External Drive
Make sure the drive:
- Has a bootable macOS installer or a full macOS system installed.
- Is formatted properly (APFS or Mac OS Extended).
- Is connected directly to the Mac (avoid hubs during booting).
🖥️ 2. Booting Procedure
🔹 For Apple Silicon Macs (M1, M2, M3):
- Turn off the Mac.
- Press and hold the power button until “Loading startup options” appears.
- Select the external drive in the list of startup disks.
- Click Continue to boot from it.
Tip: If it says you can’t boot, go to Startup Security Utility and allow booting from external media.
🔹 For Intel Macs
- Turn off the Mac.
- Turn it on and immediately hold the Option (⌥) key.
- Keep holding until you see the Startup Manager.
- Select the external bootable drive.
- Press Return to boot.
⚙️ Optional: Set as Default Boot Disk
To always boot from the external drive:
- Open System Settings (or System Preferences).
- Go to Startup Disk.
- Select your external drive.
- Click Restart or Set as Startup Disk.
🔐 Security Notes
If your Mac won’t boot from external drives:
- Apple Silicon: Boot into recovery, go to Startup Security Utility, and allow external booting.
- Intel with T2 chip: Also go to Startup Security Utility in recovery and change settings to allow external boot.