> Better to boot from an external disk?

My iMac 2015 has an internal 2TB Fusion Drive. When I upgrade from Catalina to Big Sur, would it make sense to start it up from an external SSD and move my apps to the SSD inside its Fusion Drive?

Big Sur can start up perfectly well from an existing Fusion Drive and runs from there much as Catalina does, but it will change the layout of the startup volume group to use its new Sealed System Volume. That doesn’t seem to have any effect on performance.

If your external SSD is a Thunderbolt model, then it could prove a better choice for its startup disk, but it’s not as clear–cut if it’s a regular SATA disk which connects to a USB port. Running your apps from the SSD component in the Fusion Drive would require you to split and reformat it, which isn’t an easy process as it has to be performed in Terminal’s command line. You also can’t move any apps bundled with macOS there, as they’re locked on the sealed System volume.

HOW DO I RUN DIAGNOSTICS ON MY M1 MACBOOK AIR?

M1 Macs don’t respond to traditional key combos held during startup. You need to enter Startup Options by pressing and holding the Power button until those start loading. Once the first Startup Options screen appears, press and hold Cmd+D until your Mac restarts and the Diagnostics Loader menu appears.

HOW DO I REMOVE AN OLD EXTENSION?

To remove old kernel extensions from your Mac, use its uninstaller, or follow the vendor’s instructions. If you can’t do that, manually remove those in the /Library/Extensions folder, but don’t touch copies managed by the system in the StagedExtensions folder.