Sorry to hear that your USB hard drives are experiencing disconnects via a Plugable USB 3.0 hub connected to your Synology NAS.
There are many possible reasons why your USB external hard drives are not behaving as they should, like you mentioned.
The most common issue is that many NAS manufacturers do not support USB hubs as a means to connect several USB external hard drives to your NAS.
Some NAS manufacturers do support USB hubs, but only for low power USB devices like Bluetooth dongles or USB flash drives.
For this reason, we do not recommend using any Plugable USB hubs to connect external hard drives to NAS servers, unless the NAS manufacturer themselves have declared our products as officially compatible for that type of use.
First, I would check with Synology on their official stance about using USB hubs to connect USB external hard drives like you are doing.
Next, you can try to isolate the issue by connecting each hard drive one at a time to the NAS server’s USB port directly, without using the Plugable USB hub.
If you continue to experience disconnects when the drives are direct connected, then we must assume that the issue is something besides the USB hub. I would reach out to Synology support to help investigate a cause.
If you only experience disconnects with the USB hub in between the drives and the NAS server, then another possibility is that your USB hard drives are drawing more than 25W power combined. Check with your USB drive manufacturers to confirm.
If that is the case, we suggest upgrading to a more powerful Plugable USB hub, like our 60W model USB3-HUB7C: http://plugable.com/products/usb3-hub7bc
Once again we cannot guarantee that any of our USB hubs will work if the NAS manufacturers have not declared them as officially compatible for use with USB external hard drives.
Before upgrading hubs, I would first confirm with your NAS manufacturer that they support our USB hub for this type of usage. If they confirm that our hub is supported, their support should be able to assist further.
Hope this helps!