I though I would share my experience with a DS-210j with WD6400AAKS drives.
I started with the DS-210j a few years using an old WD6400AAKS and a newly purchased one. The drives had different firmware but both seemed to work fine. After not too many months the newer one started to crash, seemingly coming out of hibernation. On the first crash I contacted Synology who analyzed my log file and suggested the drive was bad. After removing and testing with the WD test utility (which indicated no error) I was not convinced.
My usage is approximately: 15 hours a day / 7 days a week. I use it only as a file server so the drives go into and out of hibernation often.
After each crash I would check the SMART INFO, there was never any errors logged. Then I would remove the drive, install it in a PC and run the WD diagnostics. Again there was never any indication of errors. I would then delete the partition and let the volume rebuild. I this may or may not be the best practice but it is what I did.
This cycle occurred over several firmware revisions.
It seemed the same cycle would happen about every 3 to 4 months. Finally I purchased a new WD6400AAKS and replaced the one that would crash. Now I've been running for well over 6 months with absolutely no issues. The original drive has 14k hours on it and the new new drive has 2k hours.
I can't say the root cause was a faulty WD drive or some quirky interaction between WD and the DS-201j but replacing the drive it made a difference. I share this only to give others experiencing odd issues to see that sometime the tools we have to solve a problem are not adequate.