Quick Scan always works, on any drive — it finds files still sitting in the filesystem or still in Trash. That's real, immediate recovery, free to preview.
Deep Scan (raw sector recovery — finding data the filesystem has already forgotten) works fully on external drives formatted the traditional way: HFS+, APFS, or standard NTFS. This is where Deep Scan earns its name.
Two specific cases where Deep Scan can't reach the drive at all — macOS blocks raw access outright, for every app, elevated or not:
1. Your Mac's own startup disk, while you're running from it. This is System Integrity Protection (SIP), a macOS security feature — not a permission you can grant. Recovering already-deleted data from your startup disk requires booting from somewhere else (macOS Recovery, or another drive) and scanning it "from outside," the same requirement every Mac recovery tool has for this exact scenario.
2. Drives mounted through FSKit — Apple's newer, user-space filesystem framework, common for exFAT drives on current macOS. The kernel blocks raw access to any FSKit-managed volume, confirmed directly in macOS's own security log. A drive mounted the traditional way doesn't hit this wall.
1. Your Mac's own startup disk, while you're running from it. This is System Integrity Protection (SIP), a macOS security feature — not a permission you can grant. Recovering already-deleted data from your startup disk requires booting from somewhere else (macOS Recovery, or another drive) and scanning it "from outside," the same requirement every Mac recovery tool has for this exact scenario.
2. Drives mounted through FSKit — Apple's newer, user-space filesystem framework, common for exFAT drives on current macOS. The kernel blocks raw access to any FSKit-managed volume, confirmed directly in macOS's own security log. A drive mounted the traditional way doesn't hit this wall.
In both cases, Quick Scan still works normally — you're only losing the "find data that's already been deleted" capability, not the app.