Moving folders · How moving folders works
Is moving a folder to the external drive the same as backing it up?
Short answer: No. A move leaves one copy of your files, on the fast drive, with links on your Mac; a backup is an extra copy kept elsewhere. Moved files depend on that drive, so back the drive up too with Data Backup, Time Machine or another tool.
No. A move leaves exactly one copy of your files — on the fast drive — with links on your Mac pointing at it. A backup is an extra copy kept somewhere else so you can recover if the original is lost.
That means anything you've moved depends on that drive. If it fails or goes missing, the moved files go with it. Keep a backup of the fast drive too — Storage Studio's Data Backup, Time Machine or another tool — especially for folders you can't replace.
See backups.