I like the tick/check.
Thoughts
Maybe use grey for the volume icon in Finder when not all is good.
Always use grey for the volume icon (but not text) within notifications.
Maybe displace the tick, maybe drop it a little 'off the edge of the volume', just enough to have it recognisably a tick but not quite right.
Never use an exclamation mark ! or question mark ? in lieu of the tick – those alternatives are not recognisably ZEVO.
Never use amber or red for a tick – that would be counterintuitive.
Generally, allow notifications alone to serve their purpose. Allow the eye to be drawn to the text (not the green icon) in notifications.
I wonder about appearances beyond notifications only because we have the default green for the tick of the volume icon, and I begin to realise that this is not always appropriate.
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(There were other points, but I have lost this post twice due to a bug affecting phpBB. Around half an hour's work lost. Grr. Maybe I'll remember later)
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All of the above, design-oriented. Thoughts for the future.
(For problems with operating system representation of the existing icon, there are separate topics.)