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If you can’t install macOS on an external disk
You can install macOS only on disks with the GUID Partition Table scheme. Use Disk Utility to check your external disk’s partition scheme to make sure it’s the required scheme.
In the Disk Utility app on your Mac, choose View > Show All Devices (a tick mark indicates it’s selected).
Select the external disk in the list on the left.
Disks appear flush left in the list (volumes are indented below disks).
On the right, check if the description below the disk name mentions GUID Partition Map.
If it doesn’t, you need to reformat the external disk before you can install macOS on it. For information about reformatting an external disk using Disk Utility on Mac, see Erase and reformat a storage device.