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How to Set the Startup Volume / Startup Drive/Disk
When changing the startup volume, be sure to choose it explicitly or there will be a delay in system booting.
Open, then choose to set the desired drive as the startup drive.
All bootable volumes show up in System Preferences => Startup Disk*.
You might want to also disconnect or remove any older boot drives when making a switch, to avoid confusion (this is why using different names is also a good idea).
* Startup Disk is an error in terminology; it’s a startup volume (a disk can have many bootable volumes). Apple has not fixed this terminology bug in decades.
Tip: you can boot off any bootable volume at system startup by holding down the Option key; a list will be presented. Doing so does not change the default.