External drives · Media libraries and starting up from a drive
Can I install and run macOS from an external SSD?
Short answer: Yes, most Macs can start up from macOS installed on an external SSD — choose it as the startup disk (on Apple silicon, hold the power button at startup). It's a separate system. Storage Studio doesn't make drives bootable; it moves apps and folders instead.
Yes, most Macs can start up from macOS installed on an external drive. You install macOS onto the external drive, then choose it as the startup disk (on Apple silicon Macs, by holding the power button at startup to see the startup options). How smooth it feels depends on the drive and connection; a fast SSD makes a reasonable spare or test system.
Keep in mind that it's a separate system with its own apps and settings, and startup security settings can affect booting from external drives.
To get more space for your everyday system, you don't need any of this: Storage Studio keeps macOS on your internal drive and moves big apps and folders to the external one. It doesn't install macOS on a drive or make it bootable — see bootable drives and Storage Studio.