How do I change the icon on an external drive?
EDIT:
As @David Anderson mentioned, the icon is stored in .VolumeIcon.icns in the root of the external drive. This is a hidden file by default, so press ⌘ + Shift + . to see hidden files. By default, the .VolumeIcon.icns file does not exist, and if it does, the icon in the file will be used. 

If you want to see all default icon files for modification purposes, go to /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources. Then, you can move or modify any icon of your choosing to the drive.



Changing the icon of external drives still works on macOS. Here are the steps to do so.


Download an image to your computer
Open the image with Preview.app
Press ⌘ + C to copy the image to your clipboard
Select your external drive (Go to finder then Shift + ⌘ + C to see your external drives)
Select your drive and press ⌘ + I
Right-click the icon in the upper-left-hand corner of the popup window
Press ⌘ + V to paste the image to the drive