Apps · Drive, safety and recovery
Why does an app move fail with "files that differ only by case"?
Short answer: Your fast drive is case-sensitive and the app has files whose names differ only by capitals, so they'd clash. Nothing is changed; a standard (case-insensitive) APFS drive avoids it.
Your fast drive is formatted as case-sensitive while your Mac's built-in drive isn't, and this app contains files whose names differ only by capital letters. On the built-in drive they're treated one way, on the external drive another, so they would clash — and Storage Studio stops rather than risk a broken copy.
Nothing is changed. Most people never see this; if you do, the reliable fix is to use a drive formatted as standard (case-insensitive) APFS. See external drives.