Time Machine to a NAS stopped working: what now?
Time Machine to a NAS is convenient until a message like “The backup disk could not be found” or “disk not supported” shows up. Usually it is one of a few causes.
1. Is the share set up for Time Machine?
Time Machine backs up to a NAS only over SMB, and only to a share the NAS explicitly offers as a Time Machine destination. On Synology the option is in Control Panel › File Services › SMB › Advanced as “Bonjour Time Machine broadcast via SMB”; then pick the share. On QNAP it is under Network & File Services › Win/Mac/NFS. Also check that SMB 3 is allowed as the highest protocol.
2. Firmware and macOS up to date?
After major macOS updates, NAS makers often adjust their SMB services. Install the latest firmware for your NAS and the latest macOS. Read the maker's release notes: known Time Machine issues are usually listed there first.
3. Quota and space
If the share is full or its quota is reached, Time Machine stops. Give Time Machine a fixed quota of about twice the data on your Mac, and leave free space on the NAS.
4. Verify the backup
For network backups Time Machine can verify the backup: hold Option, click the Time Machine icon in the menu bar and choose “Verify Backups”. If macOS reports that the backup is damaged, a new backup is often the only fix. Rename the old sparsebundle first instead of deleting it.
5. Reconnect
Remove the destination in System Settings › General › Time Machine and add it again. Connect to the share once in the Finder (Go › Connect to Server, smb://nas.local/TimeMachine) and save the login in the keychain.
Plan B: a second backup that does not depend on Time Machine
If Time Machine keeps failing, back up your files with a second tool to the same or another share. restic, the open source program inside Lifeboat, writes to any mounted SMB share and needs no special Time Machine support on the NAS. That way your backup does not hang on a single feature.
Lifeboat backs up your Mac with restic to a disk, a NAS or your cloud storage, and restores random files every week to show you the backup works. 14 days of everything free, then 1 disk free or Pro for €39 once.
See Lifeboat