I have a similar problem with a WD20EARS-00MVWB0 (Wester Caviar Green 2Tb) drive.
The funny part is that i have 2 of them installed in the NAS, one works without problems, the other works great until i get over 50% of it filled. After that it starts disconnecting. If i delete files from it to get to about 1Tb free space it works correctly again.
The drive that works is about 1 year older than the one having problems, maybe they changed something in the production method ?
The old HDD's serial starts with "WD-WMA", the new one with "WD-WCA", don't know if it's of any help.
I'm suspecting that maybe the DNS-320 doesn't give enough power for the drive, but of course i don't have any proof for that
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Anybody had any similar problems ? anybody has any solution or idea I could try ?
P.S. the HDD that works is about 90% filled and never showed any problems, the HDD that has problems is working correctly until i get to about 48-49% filled, after that it starts disconnecting whenever i try to write files to it, but works correctly for reading. I've even reformated it (through the NAS interface).
I've took out the "bad" HDD, put it in my PC and was able to copy everything off of it, I've tried copying files on it (reformatted the drive as NTFS) and didn't have any problems filling it completely. So, the HDD itself is not broken or damaged (also, the SMART test doesn't show any kind of errors)
I'm thinking of replacing that drive with a 1TB WD Green disk i have in my computer, i rather have 3 Tb of usable space than 4tb that i can't actually use.
P.S.2 I'm still on the 2.00 version on the DNS-320, didn't upgrade since I've read there are a lot of problems with the update.
P.S.3 I've tried changing places for the 2 drives but that doesn't help.