I have a Hitachi 5K3000 drive (HDS5C3020ALA) 2TB drive in an external enclosure.
Today I installed an application and copied some data to it, and it is now almost completely full: Windows Explorer reports 34GB free out of 1.81TB total space.
For about 10 minutes it continually made a noise which was rather alarming to me. It sounded like the disk was being accessed, it would thrash for about 1 second or two, then pause for about a second, then start up again. It continued even when the drive was no longer being accessed according to Resource Monitor, and indeed even after I removed the device via "Safely Remove Hardware" this continued until I powered it off. I carefully picked the drive up while it was doing this and I could feel a slight vibration as it made the noise. Surely it was the read head moving across a large portion of its range. I also noticed that during the actual thrashing the HDD activity light was not active but it came on during the second-or-so pauses.
And so it went like this for a good 10 minutes, me not being sure if my drive was slowly dying. I installed HD Tune and was slightly relieved to see no SMART warnings. I have no means of backing up this much data: It would never be worth the cost. So if it was going to die, the data would die with it.
And then all of a sudden it just stopped. Blissful silence.
I run Windows 7's disk defragmenter utility, and analysis reports 0% fragmentation on this 98.2% full drive.
So is this the explanation? Modern hard disk firmwares now defragment automatically? Does this mean that disk defrag software has now become completely obsolete?