This is related to the firmware of your laptop or the controller in external enclosure. I for example know that my T530 is unable to detect my USB 3.0 HDD enclosures and thumb drives and let me boot from them after powering on the laptop, but with a warm reboot everything works. Another example is an external dock that doesn't play nice, partitioning some drives in it is different than connecting the same drive internally. From that experience, I'd say try another computer and another enclosure or install on the target machine. This should save you time spent chasing firmware bugs.
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