Delete Old Time Machine Backups to Recover Space

Please NEVER EVER delete Time Machine backups to increase space, all this does is make Time Machine not work. If you need additional space then increase HD capacity by replacing with a higher capacity HD OR format the Time Machine drive and start over.

just as rkaufmann87 says, if you do manage to delete a backup as of a certain date, the files in it (which may be the ONLY copies in the entire backup Set) will be LOST, and your backup SET will lose its integrity.

When Time Machine needs space, it automatically creates ispace by consolidating backups, not by deleting files.

You can also add an additional backup destination on a different drive, local or remote. When you do this, each is a stand-alone backup set. Every other backup will go to every-other drive.

What’s tripping you up here isn’t permissions or syntax, it’s how Time Machine behaves with network backups. On a Synology, Time Machine stores everything inside a sparsebundle, and tmutil can’t delete individual backups unless you point it at the mounted backup path plus a specific timestamp.

Running tmutil delete by itself or passing a snapshot name won’t work, which is why you’re seeing that error. The bigger issue though is that manually deleting backups inside a sparsebundle on a NAS often corrupts the backup set, especially on newer macOS versions, and Time Machine may silently stop trusting it.

The only safe ways to reclaim space on a NAS are letting Time Machine prune old backups on its own, increasing the quota on the Synology, or blowing away the sparsebundle and starting fresh. If you truly need to surgically remove old backups, you’d have to mount the sparsebundle read-write, identify the exact backup path under Backups.backupdb, and delete by timestamp, but that’s risky and not something I’d recommend unless you’re prepared to lose the entire backup history.