The internal '1st HDD' is always going to be 'a'. The first connected USB storage device will be 'b'. But even without that, your output shows just two drives, and one is 465.8 GB in size. Pretty sure that isn't your USB.
And, you can always remove it and rerun lsblk to see which one isn't listed anymore.
And, you can always remove it and rerun lsblk to see which one isn't listed anymore.
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