The problem stems from having changed the icon in the past prior to updating macOS. I did this years ago when the Macintosh HD started being an SSD. The HD icon just didn't cut it for me anymore. Now after having updated to Tahoe it retained my custom icon and I can give it another new icon via the Get Info dialog just like you could for decades, but I can’t delete the icon to get the new default one. It just does nothing if you try.
I may not like the new default icon anyway, but at least wanted to take a look at it first. So getting the default from some location in the System folder and pasting it is the only way to go.
I have yet to find it though. It seems to not reside in /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources
EDIT: Found it: /System/Library/Extensions/IOStorageFamily.kext/Contents/Resources/Internal.icns
I am currently tempted to use the Floppy Disk icon from /System/Library/Extensions/IOSCSIArchitectureModelFamily.kext/Contents/Resources/Floppy.icns
instead… 🙃