Well, WD Engineers found the root cause after about 6 months of poring over logs and analyzing the symptoms.
The problem is caused by a large external drive connected to the NAS via USB containing a lot of content. After the NAS has started up, it will try to index the content on the external drive. If there is a lot of content, the unit will run out of RAM after some time. First dashboard and cloud connections will become unresponsive (connection refused), then the file serve will quit, and the media server will shut down as well as the SSH connection. Upon reboot, the unit will respond for a short while until the indexing resumes.
Disconnecting the USB drive and power-cycling restores full functionality. The insidious thing about this issue is that it creeps up on you as the external drive fills up. Form me, the issue started when I transferred a lot of backup files from the NAS to the external drive, about half a year after I had purchased and connected the 4 TB USB drive.
Here is the official response from WD, for the record, The suggestions from them are not an option for me, so i just disconnected the external drive for now. Also, it really blows that there is no way to just stick more RAM into the NAS. Well, I wish I had known these things before I bought the darn thing, now I have to live with my choice.
— Response from WD support —
Due to the fact that you have a USB Drive attached to the EX2 Ultra device, the scanning and indexing services running within the CLoud try to index the content of the USB drive and even generate thumbnails for the media files. This eventually causes the device to run out of RAM memory and run a command that renders the device unreachable to keep in running despite the no-memory situation.
When content is added to the external USB drive, the content is indexed so the data is accessible in Web and mobile app
however we do not generate a thumbnail. To generate a thumbnail, “USB Content Availability” has to be enabled and based on the System Logs you indeed have this enabled. Extracting the thumbnails uses more memory and CPU to process the files for remote access.
SOLUTION:
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Safe Eject the external USB Drive and disconnect
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Turn OFF Remote Cloud Access for now. We will enabled it later on
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Turn OFF iTunes Server for now.
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Turn OFF Twonky DLNA Media Server for now
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Turn OFF “USB Content Availability” to conserve memory on the device.
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Reboot the EX2 Ultra
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Connect the External USB drive
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USB Backups and Restores should no longer trigger the Out Of Memory shut down and the unit should not lock up as before
•You can enable remote access, however, the crawler will index every new piece of data added to the external USB drive which increases the memory usage. It would therefore be advisable to turn off Cloud Access when the External drive is connected.
•We would not recommend to enable Twonky and if you do, just disable Media Serving on the USB drive so that Twonky is not indexing USB Backups.
•We would not recommend to enable USB Content Availability. However, if you require thumbnail access to your USB drive via Cloud Apps, this is the only way which will increase CPU and memory usage.
