To boot a Mac from an external hard disk, first make sure that the hard disk is equipped with a complete and correctly formatted macOS system, such as a boot disk made by "Migration Assistant", "Disk Utility", "Terminal" command or third-party tool; secondly, connect the hard disk to the USB-C or Thunderbolt interface of the Mac under shutdown; then immediately press and hold the Option (?) key when powering on; then use the mouse or keyboard to select the external hard disk to boot; if the recognition fails, check the cable and power supply; after successful startup, you can perform system repair, data migration, test new macOS or clean files, etc., but avoid unplugging the hard disk during operation to avoid system crash or data loss.

Booting a Mac from an external hard drive is actually easier than it sounds, but it does require a little preparation. This is useful if you need to use another system's macOS to repair the current system, migrate data, or try a new version of macOS without affecting the main system.

Make sure the external hard drive is bootable

It can't be used to boot a Mac by simply plugging in a mobile hard drive. First, you must confirm that the hard disk contains the complete macOS system and the format is correct (for example, using APFS or Mac OS extension format).
Common situations include:

• You have created a boot disk using Migration Assistant or Disk Utility
• Create a cloning system using the Terminal command or a third-party tool such as Carbon Copy Cloner
• Apple's official recovery partition or installer can also be used as a temporary boot disk

If you are not sure, you can plug it into your Mac first, and then go to the "Boot Disk" preferences to see if there is an optional system displayed.

Restart the Mac after plugging in the device

Turn off your Mac and plug in the external hard drive (it is best to plug it directly into the USB-C or Thunderbolt interface of the computer itself to avoid using the docking station to increase success rate).

Then press the power button to turn on, and immediately start pressing and holding down the Option (?) key after hearing the "dong" sound. Some new M1/M2 chip Macs may not require buttons, but will automatically enter the startup management interface. However, for the sake of safety, manual operation is still recommended.

Select the external hard drive in the boot option

At this time you will see a boot management interface, which lists all available boot disks. You can see the built-in hard drive and the external hard drive icon that just plugged in.
Use the mouse to select the external hard drive item, or use the keyboard arrow keys to select it and press Enter.

If only one external hard drive is connected, it is usually selected by default. However, it is also possible to encounter unrecognized situations. At this time, you can check whether the cable supports data transmission and whether the hard disk is powered normally.

Things to note after entering the system

Once successfully booting into the external system, you will find that the system runs as usual, but the speed may be affected by the hard disk read and write performance. For example, if you use a boot disk installed with an ordinary USB disk, the response will be slower; while if you use a high-speed SSD, it will be much smoother.

At this time you can do the following:

• Check and fix problems with the main hard drive
• Use Migration Assistant to migrate old data to new machine
• Test whether the new version of macOS is compatible with your software
• Clean up files that are not accessible to the main system

It should be noted that: do not unplug the external hard drive at will , otherwise it may cause system crash or data loss.

Basically that's it. Although the steps seem a bit too many, the actual operation is not complicated. As long as you prepare a bootable external connector, there will basically be no problems.

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