A further update.
I can confirm that USB Backups are working correctly, but I have seen a number of instances of very high “sys” usage when a USB drive is attached. I have tracked this down to 2 culprits
The twonky server - which was a configuration issue
the /usr/local/bin/wdnotifier process. This process seems to wakeup when a USB Drive is connected and locks into high “sys” usage which makes the unit very unresponsive. Killing the process instanstly frees up the unit. As an example, I was doing a backup to en external USB and the system was barely responsive. The highest CPU consumer was the wdnotifier process and “sys” was around 95%. The rsynch (backup) processes were barely getting a look in. killing wdnotifier brought “sys” down to around 45% and I started to see some idle (not much) - and the backups then proceeded at a much faster rate.
I have had a google on wdnotifier, and find that my experience is quite common, and the solution that I have adopted is the accepted one until WD fix the problem,
What does WDNOTIFIER do???
I’ve had a look at the conf file for wdnotify, and found the following
exclude_dir=^System Volume Information$
so it would seem that an entry alond the lines of "exclude_dir=^NAS Backups$
To test, I had the unit stable, and had completed a trial backup. I had killed wdnotifier and verified that when I plugged the USB into the EX4 there was no notification that the USB had been connected (confiring my working theory that WDNOTIFIER watches for USB connections). I then started WDNOTIFIER and confirmed that the USB drive was “noticed” and connected. However the unit then became unresponsive with the usual “disk lights flashing” scenario. I was connected via SSH and could see WDNOTIFIER spike along with high “sys” CPU. SSH into the unit became unresponsive. The GUI was barely responsive and ejecting the USB made it unresponsive (in “Updating” mode)
All very familiar.
Restart the unit wihtout USB attached.
Attach the USB drive with two SSH sessions open. Both twonky and wdnotifier spark up, “sys” CPU drives to over 90% and peggs there. Unit becomes unresponsive.
Wait 5 minutes. No change.
Kill WDNOTIFIER whilst leaving twonky, the unit comes back to life within 20 seconds, “sys” CPU drops to below 10%m twonky stays streaming and a backup can proceed without any issues.