I have a WD Caviar Green 1Tb HDD, around 2 years ago. It was the data drive in my desktop computer, when I upgraded I took out and put into an external rack and I'm using to carry data.
After several weeks, the HDD started to work "alone". I mean, it can be heard clearly that it's writing or reading, at least the head is moving and makes periodically a series of movements. Around 2 movement groups in a second. After doing this 5-10 minutes, it goes silent. If it's used (I read or write something), it does without delays.
It's definitely not software. If I safely remove from the system, the HDD continues to work alone. Even if I unplug the USB cable and I leave only the power supply cord.
First I thought it's caused by bad sectors so I wiped the drive by writing zeroes (a diagnostic tool from WD site). The same.
This wouldn't be a big problem if I don't hear my second HDD doing the same.
In the desktop computer now I have a WD Caviar Black, 1Tb and the other day I copied some more data by accessing randomly a lot of files so the HDD head moved a lot. When finished, this also started to work alone. (Note that when I analyzed the disk activity with the Resource Monitor (Win7 x64), there was absolute no usage of the HDD). After doing his job, the HDD went silent. Nothing after that.
It is possible that WD has some king of 'self-maintenance' or 'bad sector discovery' or 'built-in defragmenting' system? Or simply the HDD is just too tired? Or it's going to break down?
It seems more likely that something like the Search Indexer service is reading the drive when it is present and ignoring it when it is absent. Another good candidate would be antivirus software doing a background scan. Or malware looking for your credit card numbers.
Do you have drives that _do_not_ do this, from another manufacturer? If not, I'm inclined to suspect a software service that touches all drives that are present.
You can install a utility that tracks disk IO, or at least writes, but the amount of log to wade through can be amazing.
There's been some discussion about this on the WD Community forums. It seems like a pretty common problem with WD drives. Unfortunately, I don't think anyone has found an actual solution. The general consensus seems to be that it's some kind of 'self-maintenance' that the drive does when it's idle.
I also have a caviar black (WD1001FALS) that does this exact same thing. If I, say, record a long FRAPS video, it will make loud noise for a few minutes AFTER the recording finishes. If I do anything with the drive, the noise disappears, but returns again after the drive returns to idle. I also have a Samsung F1 hard drive, and it does not do this in any scenario.
Luckily, except for the unnecessary and annoying noise, I have not noted any other problems with the drive. The 'problem' has persisted for over a year now, but the drive is still in 100% condition, not a single bad sector or anything.
Sorry to bump an old topic but just wanted to post that I did noticed the same behavior on two WD20EARX that I have on an unRaid server, it's easy to notice it by looking at their temperature, they are usually at near 31ºC when idle (not spin down... I have unraid currently set to never spin down...) and they reach near 38ºC when doing constant read/write for some time... but I did already noticed the thing at least twice, in two months, temperatures on my two WD20EARX (and only really on these two disks, not on a WD20EFRX - WD RED...) would suddenly start rising exactly like if it was doing read/write, and system wasn't doing anything for sure, no unraid read/write counts increasing... and no activity at all on the HDD led on the PC case... after some time temperatures go down to normal idle ones again...