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On a Mac, when the file dialog pops up, click on the left pane and select "Computer".
In the right pane, right click to get the context menu and enable "Show Hidden Files".
Then you will see the Volumes and can select external drives.
I have no idea why the macOS now decided to call Volumes hidden. Makes no sense to me but ...
Here is a screen shot of Sigil on my mac:
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