Fast forward several months....   I've since upgraded the firmware on our DD appliances from 5.7 to 6.2 (via 6.1) successfully, and upgraded Networker from 18.2 to 19.2   I tried this same process again today, and it worked immediately, with no suspect devices flagged - so it's now just a process of some further testing, switching backups to new storage nodes and waiting out the retention times I strongly suspect it was the very old DD OS that was the source of the original issue, but I'm very happy the issue is fixed regardless of the cause.
We have a Networker 18.2.0.1 install with 1 central server and 2 Storage Nodes.  The central server is now running Windows Server 2016, but the storage nodes are still running Windows 2008R2. The storage nodes are currently physical and serving double duty for both DataDomain and physical tape libraries and as part of the process we intend to seperate the DD hosting to a virtual machine.   This morning I tested creating a new device on the existing Datadomain from a new Windows 2019 VM, which worked and I was able to carry out a backup to it, but caused all of the DD devices on the existing Storage node to flag as 'Suspected'. I've tried to confirm if a DataDomain appliance can only be accessed by a single Storage Node, but my google skills have failed me.  Can anyone confirm this, or should it have worked but maybe I've done something wrong?   Also any general advice on how I might tackle this upgrade/migration would be very welcome. I previously upgraded/migrated the central server by building a new VM, copying the networker folders, and renaming and ReIPing everything the same but was sure there was a less drastic way for the storage nodes.
Storage Node OS change