From the knowledge base, here are the safe, documented checks and cleanup steps:
1. Check storage usage in Storage Manager
- In DSM 7.4, go to Storage Manager > Overview to see overall system status, volume usage, and drive information.
- You can also go to Storage Manager > Storage to view and manage storage pools and volumes. [11]
2. Identify what is consuming space
The KB points to these common space consumers:
- Synology Drive Server
- Snapshots
- Download Station
- Active Backup for Business
- Recycle Bin
These are listed as common reasons storage usage may be higher than expected. [10]
3. Free up space safely, depending on what is using it
- Synology Drive Server: refer to the linked article in the KB to free up storage space. [10]
- Snapshots: refer to the linked article to free up snapshot space. [10]
- Download Station: refer to the linked article to free up Download Station space. [10]
- Active Backup for Business: delete backup tasks that are no longer needed, or set retention policies to remove older backup versions.
Do not manually delete Active Backup for Business data from File Station, because it will not free space and may cause inconsistency. [12]
- Recycle Bin: empty it. The KB notes that files not permanently deleted still take up space, and freed space may not appear immediately. [10]
4. If space still does not return after deleting files
- On a Btrfs volume, DSM uses a background task to reclaim storage.
- In DSM, go to Storage Manager > Storage > Global Settings and make sure a Space Reclamation Schedule is configured.
- Wait for background processing to finish.
- If space is still not released after a long time, use the Restart option from the person icon in the upper-right corner of the storage system desktop. [13]
5. Storage overview on older DSM
On DSM 4.3, Storage tab shows free/used volume space and disk status. The guide lists disk states such as Normal, Initialized, Not Initialized, No Disk Installed, System Partition Failed, and Crashed. [1]
If you want, I can also summarize this into a short step-by-step checklist for DSM 7.x only.
Sources used: [10], [11], [12], [13], [1]