That all depends on the File System of the device used, not a Sigil issue.. On Windows and most macOS machines, file paths are case insensitive but case preserving. On other macOS and most Linux boxes, file systems are case sensitive.
So you are probably using Sigil on a machine with a case-insensitive file system. While your ADE device is case sensitive.
The best way to detect these issues in your epub is to use the epubcheck plugin.
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