External drives · Checking your drive
What does "An app has started keeping its own copy of files we'd moved to your fast drive" mean?
Short answer: An app replaced Storage Studio's shortcut with its own folder, so there are now two copies — one on your Mac, one on the drive. Storage Studio leaves both untouched until you decide which to keep.
An app replaced the shortcut Storage Studio left with a real folder of its own — so there's now one copy on your Mac and the moved copy on the fast drive. Apps sometimes do this after an update or a reset. Storage Studio leaves both copies alone rather than guessing which one you want.
Check which copy the app is now using — usually the new one on your Mac. Once you are sure you no longer need the older copy on the drive, you can remove it; until you decide, nothing is deleted.