When I boot iOS I get "the disk you inserted was not readable by this computer"

WD External Drives External Drives for Mac
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I did a search and found 1,000 bad answers.

I leave my My Book Duo attached to my iMac at all times. When I boot my Mac I get the message “the disk you inserted was not readable by this computer” I click ignore and my My Book Duo mounts and works fine. Why does this message pop up every time I boot? it’s very annoying.