Finder sidebar icons showing as circles for disks
Do you have WhatsYourSign (program that checks if an app is signed) installed by any chance? I removed that and the icons are back to normal
"...looking at ~/Library/Preferences/com.apple.sidebarlists.plist but this doesn't seem to exist in Mojave"
In my Mojave this plist exists!
Did you already "reset" the Finder?:
- click the little apple icon on the top menu bar
- select "ForceQuit"
- select Finder in the list that pops up
- click ForceQuit.
This issue might also be related to Finder Extensions.
It was solved for me in macOS Big Sur 11.1 by going to System Preferences → Extensions → Finder Extensions and disabling Finder Extensions. Then relaunch the Finder:
- press and hold the Alt/Option key
- right-click the Finder icon in the Dock
- select Relaunch
This made the Finder sidebar icons show up as external disks again.