These instruction will let you backup a directory from the top level of volume1. That is where the shared folders appears.
In the DSM control panel, find Terminal & SNMP and click it. Then in the “Terminal Tab”, check “Enable SSH service” and pick a port number. Do this for both Disk Station Devices.
This is applicable to DS918+ and DS920+ and likely very similar on other Disk Station Managers (DSM). The two systems are currently on DSM 3.7.1-86003 Update 1. To copy a directory from one Synology device running DSM 7 to another, you can use the Shared Folder Sync feature, which allows you to replicate the directory while maintaining permissions and ownership. Ensure both devices are on the same network and set up the sync task in the Control Panel under Shared Folders. In Ctrl Panel, find File Services and click it. Then check “Enable rsync service” and put in the SSH port number that you normally use. Click Apply. DO THIS FOR BOTH DEVICES.
The copies will operate as a “push” system, so the instructions and tasks should be located on the DEVICE that we are saving from. In the Control Panel, still under File Services, Click the Advanced tab, and scroll down until you find “Shared Folder Sync”.
Now we will set up a Backup Task. In the Shared Folder Sync Area, click “Task List”
A popup box will appear. Click Create.
A box called Create a sync task. Will appear. In the blank type a task name. Then click Next.
Yet another box will appear. Select a source to sync, click the I understand, then click Next.
Now a box pops up to select the sync destination.
Fill in the server IP addres or name . Use the dropdown and wait for it to populate.
Put in the user name and Password for the destination device.
Test the connection if you want.
Then click Next.
A popup will appear to decide when the backup should run. Assume you want to schedule it every day at some time. So choose Advanced schedule.
Also, choose a run time. It is military time. 00:00 is midnight. Now choose OK.
When the box closes, you will be returned to the Set up schedule. Click Next. Then Confirm settings by clicking Done.