What happens if the files of a moved folder go missing from my external drive?

Storage Studio tells you, with a way to fix it. It checks the folders it has moved a few minutes after launch, every few hours and when your drive is plugged back in. A drive that's simply unplugged is never a problem — you hear about it only when the drive is connected and the files still can't be found.

• If the folder is in the drive's Trash, Put it back returns it in one click and every app finds its files again.
• If it's gone, the alert names the app affected, and the Moved files window explains what you can do: restore it from the drive's Trash or a backup and click Check now, or Remove broken links…, which removes only the links that point at nothing so the app can rebuild what it can.
• If a folder was moved twice, Repair links points it straight at its files.

Choose these alerts in Settings → Notifications.

See moving the same folder twice.