mchapman-
I just had the same issue as you on my new iMac, and came across your post while trying to figure it out.
Swapping the cable from a USB port to a Thunderbolt port (with a lightning bolt symbol), solved it for me. I'm also using an Insignia (Best Buy brand) USB-C adapter cable.
I know it's been a while since your post, but I hope this helps!
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