External drives · Drive formats
Should I format my fast drive as case-sensitive APFS?
Short answer: Use plain APFS unless you specifically need case-sensitive. Your Mac's internal drive is case-insensitive, and a few apps contain names that differ only in capitals — Storage Studio checks for those and won't move an app that would clash.
Choose plain APFS unless you have a specific reason not to. Your Mac's internal drive is case-insensitive, and keeping the fast drive the same avoids surprises.
On a case-sensitive drive, two files whose names differ only in capitalisation (for example Readme and README) are different files. A few apps contain names like that, and Storage Studio checks for them before moving an app: if the move would cause such a clash, it stops and explains rather than moving the app.
If you already have a case-sensitive drive and it works for you, there's no need to change it — Inspect shows the drive's format so you can check.