The fact the errors occur before the "reboot" part of the installation tells me it isn't a driver issue. The "unable to set startup disk, SDErrorDomain 104" suggests the error is a check in firmware preventing this.
It's probably simply unimplemented, and rather than try anyway there is a check somewhere in the firmware that the drive being marked bootable meets some criteria. Clearly the internal drive meets that, and some external drives do. Running a tracer on the commands sent to the drive would likely identify it.
It's probably simply unimplemented, and rather than try anyway there is a check somewhere in the firmware that the drive being marked bootable meets some criteria. Clearly the internal drive meets that, and some external drives do. Running a tracer on the commands sent to the drive would likely identify it.