Most instructions founds online are for older DSM versions (6). The following steps were performed by a synology support agent during a support case I created.
I have a DS1821+ and during a recent storm I lost power. I have a UPS however it failed to shutdown the synology.
NOTE: Make sure and test your UPS shutdown, if you have many disks it can take longer to shutdown than reserved battery capacity.
After the power down the DSM came back and had a warning on the main volume.
The following steps were obtained by looking at /var/log/bash_history.log. Before you try any of these steps, attmpt to check the filesystem in Storage Manager, these steps are only if you fail to unmount.