A Time Machine destination can reach its quota while the NAS volume still has free space. The relevant limit may belong to the backup account or shared folder, so the volume's capacity display is only part of the investigation.

Match the Mac to the destination account

Identify the shared folder advertised for Time Machine and the NAS account used by this Mac. Synology documents the Time Machine folder selection under File Services, Advanced, Bonjour. Keep the account identity with the destination path in the backup record, without recording its password.

Inspect the limits that apply to that account and folder. DSM supports shared-folder quotas on Btrfs-capable models; user quotas for individual shared folders also depend on Btrfs support. On ext4, Synology's guidance uses user quotas at volume scope. A quota affecting other data on the same volume needs different planning from a folder dedicated to one Mac.

Compare the effective limits with the workload

Record the configured limit, current use and available volume space. If several Macs share one destination account or folder, establish which backup sets contribute to the total before changing the limit. A full quota does not identify which computer generated the latest growth.

Estimate the next backup's changed data as well as the initial backup size. Synology warns that a backup can fail when the remaining quota is smaller than the changed data. A fixed multiplier is a planning guideline, not a guarantee for every pattern of large media or virtual-machine edits.

If capacity is increased, check that the volume can support the intended allowance alongside other shares and reservations. Then run a normal backup and inspect its completion on the Mac. Keep evidence of the NAS limit and the Mac's result together. Avoid manually deleting files inside a Time Machine backup set to force it below the quota; use the supported backup-management workflow and preserve the recovery dates the owner still needs.

Sources: Synology documentation 1.