the clicking noise happens every 20 seconds?
20 seconds was a bit of an overestimation - although with your ears alone it's trickly to get right!
The timer on the drive is set to 8 seconds as default, but your frequency of access determines what happens.
-8 seconds passes with no access
-some time passes. Could be just 2 seconds if your OS/app is accessing every 10 seconds
- heads unload
- cycle repeats.
Is this the only WD drive that does this?
I think all of their green range do (anyone disagree?).
Though, the sneaky bit is that early drives correctly log the number of times this is happening through SMART. When people started getting concerned, WD disabled the logging to the SMART parameter, so my drives don't accurately tell me how often this is happening.
Is it very audible such as a mis-read?
Difficult to say really. It's a "different sound" so while it certainly isn't loud, you notice it. Sounds like something small unhinging - not that my description helps at all.
Is this HDD suitable for running 24/7 in your opinion?
Take a look at my response to Karl in this thread. Really depends on usage
- for occasional access it's great and will probably save you a lot of power
- for frequent (but with accesses more than 8 seconds apart by default) accessing like a Linux OS/App writing every 10 seconds, you wont save a lot of power and the drive will be far more mechanically active.
It's worth me adding that whilst I am frustrated with WD about their silence on this issue, the drives have been faultless (so far!).
Hope that helps,