Why Does a NAS Report More Used Space Than Shared Folders Contain?

A NAS reports more used space because the volume and the shared-folder view are counting different storage scopes.

Shared-folder totals usually walk visible files. The NAS volume may also count snapshots, recycle bins, application datasets, filesystem metadata, reserved space, deleted files still held by a process, and data hidden below a mount point. Reconcile those layers before deleting anything.

The NAS and the File Browser Count Different Things

Start by confirming that both numbers refer to the same mounted volume and the same moment. Tools that show filesystem-level and directory-level measurements answer different questions: one reads allocation counters, while the other totals reachable paths.

A small gap can come from allocation blocks, metadata, reserved capacity, sparse files, compression, or rounding. A gap of hundreds of gigabytes usually needs a named consumer rather than the blanket explanation “filesystem overhead.”

Snapshots and Recycle History Retain Old Blocks

Deleting a live file does not free its blocks if a snapshot still references the older version. In one measured ZFS case, snapshots keep deleted blocks referenced and produced a large gap between dataset reference size and total used space.

Snapshot interfaces can also make single-snapshot sizes look smaller than the space freed by removing a range because blocks are shared among versions. On ZFS, USED, REFER, and AVAIL describe different scopes. Recycle bins add another visible or hidden retention layer, and a snapshot can retain files that were already placed in that bin.

Open Files and Mounts Can Hide Active Usage

A process can delete a log or temporary file while continuing to write through its open file descriptor. The path disappears from shared-folder totals, but deleted files that remain open keep their blocks allocated until the process closes them.

Mount ordering creates a different blind spot. If files were written into a directory before another filesystem was mounted over it, normal scans see the mounted content rather than the underlying files. A documented df-versus-du investigation traced missing usage to data hidden below an active mount point.

Reconcile the Difference Without Deleting Blindly

Record volume usage, dataset or share usage, snapshot-exclusive usage, recycle-bin totals, application datasets, open deleted files, mounts, and filesystem reservations. Use the NAS or filesystem's own accounting for snapshots and copy-on-write datasets instead of forcing every number through one directory scan.

Estimate reclaimed space before pruning versions, stop or restart only the process holding a confirmed deleted file, and inspect underlying mount paths during a controlled maintenance window. Keep retention policy in view: snapshots can retain already-corrupted data, but removing them casually can also remove the only usable rollback point.

FAQ

Can filesystem overhead explain a very large space difference?

It can explain a modest, stable gap, but not every large one. Hundreds of missing gigabytes more often point to snapshots, hidden datasets, open deleted files, recycle history, or data beneath a mount.

Why does emptying the recycle bin not immediately free space?

A snapshot may still reference the deleted blocks, a process may hold a file open, or the filesystem may reclaim space asynchronously. Check those layers before repeating deletions.

Will rebooting the NAS make the missing space return?

Only in cases such as deleted files held by running processes. A reboot does not remove snapshots, recycle-bin contents, filesystem reservations, or data hidden below a mount point.