Looks like the Open Media Vault appliance I had running via the HP N40L Microserver had some issues.
I left it overnight to copy 400+Gb of files and woke to find it had failed half way. Host was up, but networking wasn’t working quite so well. Rebooted and immediately saw the root file system failed to mount and caused the usual panic.
I suspect the USB key I had the OS installed on has corrupted. Which means my data should be okay, although hasn’t impressed me much.
Think I will go back to my Debian install and just do everything via it. So it’s not an appliance, but at least I know it will work day in and day out when ran from the hard drive.