Thank you Eau Rouge.
But it didn't work.
I opened the RuggedUSB-C_TBT.icns file with Preview (downloaded this file from LaCie).
Clicked Edit>Select All, clicked Edit>Copy.
Then I went to the Get Info panel for the external hard drive and highlighted the icon image that I want to replace in the upper left (it was highlighted with a blue border).
Then I clicked Edit>Paste. Nothing.
Went back to Preview and clicked Edit>Select All, clicked Edit>Copy again.
Went back to the Get Info panel and made sure the icon image was selected. Clicked Command>V. Nothing.
Something is making what used to be very simple now very difficult. And frustrating.
Would appreciate any more suggestions.