WD Elements 2Tb drive 'seeking' all the time

One of my WD drives is consistently accessing the disk, even when it’s not in use.  It makes a muted clicking (not a ‘click of death’, this is the sound it would normally make when accessing data) all the time it’s not in use - ironically it’s quieter when asked to access data…

I have 5 other WD Elements (4 other 2Tb and one 3Tb) so I’m pretty familiar with the drives and the noises they make.  This is the only one out of the 6 which is doing this.  I’ve run HD Tune to check the disk health and it’s come back fine; no damaged sectors, seek/read/write/latency all OK etc. I’ve subsequently found WD’s own tool which is running as I speak but it passed the short test (including SMART) no problem.  It’s running the Extended Test.

The drive seems to still be functioning OK and I’ve not suffered any data loss (fingers crossed I haven’t just jinxed it!).  I’ll wait to see what the outcome of the extended test is, but has anyone else encountered a drive being active when it should be idle?  It’s connected to a USB hub with another 6 drives and none of the others are doing this.  Even forcing it into standby using WD Spindown only silences it for a while before it spins back up.

I’ve looked at open applications to see if there’s something polling the disk and preventing it from going into standby but nothing obvious stands out.  My first suspect was my AV but it doesn’t appear to do background scanning so I don’t think it’s that.

If the Extended Test comes back with a fail for some reason I guess I’ll have to RMA it (it’s still under warranty).  My only query is that it is full (40Gb free of 1.8Tb).  I had a Seagate fail under warranty years ago and Seagate were good enough to send out a replacement straight away so I had somewhere to try and recover to - I then packaged up the drive to be returned in the packaging they sent the replacement in and sent it back a couple of days later.  Do WD do similar?  I don’t think I have enough free space across the other drives to be able to shuffle the data about in full.

Fingers crossed the Ex. Test comes back OK, perhaps I’m worrying about nothing…

Interesting, the drive scanned OK (which is good news!) but I’m still perplexed by what it’s up to…

Test Option: EXTENDED TEST
Model Number: WD Ext HDD 1021
Unit Serial Number: WMAY01 ******
Firmware Number: 2021
Capacity: 2000.40 GB
SMART Status: PASS
Test Result: PASS
Test Time:

18:32:26, June 12, 2013

Anyone with any ideas?