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Not Dead Yet: WD Releases New 6TB 2.5-Inch External Hard Drives - First Upgrade in Seven Years
(www.anandtech.com)
This is a most excellent place for technology news and articles.
My bet is on density. You cram so much in such a tiny space, so any tiny imperfection or fault will corrupt the data or render the drive unusable.
At the time it was fine. I had an array of 4tb drives that I was backing up with a series of 5gb drives. They were just so unreliable; all but one failed while the array they backed up is still spinning strong.