I've been experiencing the same issue with TS5410R units after upgrading to firmware version 5.16-0.18. In my case, the TS5410R is configured as an iSCSI volume for harden Veeam repositories. Basically the appliance just stops responding to everything (including pings), and we have to have the end user power cycle the units with the power button or by pulling the power cable as they completely unresponsive. I see firmware 5.34 is available now, but based on the release notes, I'm not confident that will fix anything.
dcc