After discovering that my hard disk was still under warranty and having no success at navigating their website to send an email (which seems to go around in circles), I telephoned them and spoke to a very helpful advisor who opened a replacement RMA ticket. A week ago the drive arrived, I put it in an external hard disk box to see how it behaved. It ran and didn’t spin down every 22 seconds!
I tested it for quite a while - more than 24 hours, and I made an extended audio recording of it just to make sure. Nope. It stayed running, if I told it to spin down using the Keep Drive Spinning.app it spun down and stayed spun down. If I woke it up, it woke up and stayed woken up. So I made a full clone of my existing system, opened Terminal and unfused the fusion drive diskutil cs delete [Logical Volume Group], then erased it with the write zeros option and then replaced the drive with the new one from WD. After remaking the fusion drive and cloning everything back again the Mini is running like a dream now, perhaps even better than before now that I have more free space available. Problem solved.
When I spoke to WD before they were quite convinced that this particular model did not have the IntelliPower feature and didn’t spin down by itself, but here at the WD users forum there are several other people who have examples of that drive and say that theirs do. Perhaps there was a short period of time when they did actually let some out with that firmware, however this one doesn’t and so it’s all sorted now!
The disk label shows the same disk model number, I assume the firmware must be different, even though it reports the same firmware version (Revision: 01.01A01).
Since the RMA was opened on 17 February, I found it quite surprising (and reassuring perhaps) that the date of the refurbished drive is only the day before! However something went wrong with the delivery, it took almost two weeks before it actually arrived though the documentation said it was shipped from Budapest on the 17th. I phoned them and told them it hadn’t arrived yet and then two days later it appeared in my postbox.