I muddied the waters with my Mac Mini media server at home. I installed the latest updates from Apple. The Mac Mini is now running OS X 10.10.3. That update was big, had a lot in it, and it took about an hour to apply.
And while I was at it with all the rebooting, I decided to do the hardware test. When the OS had stabilised and there was no more updating, I rebooted again and kept my finger pressed on the D on the keyboard. Didn't work. Bluetooth keyboard. Replaced it with a wired keyboard, did it again, that worked. Came up with Diagnostics. I ran the simple test first - no problems found. I ran the extended tests, and that took about an hour and a half. No problems found. Hmm. The diagnostics tell me that the hardware is fine.
So maybe all my problems are with the software. I booted it up in Recovery mode, ran Disk Utility and repaired the Macintosh book disk. Just making sure there are no residual problems. When I had done everything I could possibly to make sure the system is clean, I rebooted normally and let it sit for a while. Looked good. Started rsyncing data again, and it's gone well, but not perfectly. I haven't had a grey-screen event since the upgrade, but I've had two other issues - runaway processes, and update issues.
I keep the Activity Monitor running so I can try and see what's going on. The Mac Mini can sit there for hours doing nothing at all, just chilling. And if I try to get to it via my Linux box, like vnc to it, or ssh to it, or rsync to it, sometimes the connection is rejected. When I turn around and look at the Mac Mini screen, I can see on the Activity Monitor that there is a kernel_task consuming 98% of the CPU. What is it doing? I don't know. I can't kill that kernel_task, I can't really do anything. Sometimes the mouse and keyboard are responsive, most times they just ineffectively move a twirling ball around the screen. Only thing I can do to get control of it again is hold the power button down and wait for it to power down. Then reboot, clean, try again.
When the Mac Mini is just passively sitting there, the Activity Monitor should show no processes using more than 1% of CPU. Nothing should be happening. But stuff is happening. I have noticed in the Activity Monitor that another process will often zoom up the ranks and start consuming 98% of the CPU, and that's the AddressBookSourceSync. I know it sits in the background, like the Calendar, and regularly check for updates to the Contacts. It's normal to do that, but it's not normal to consume 98% of CPU while doing it.
And I still have an issue with updates. I noticed it when I started thinking that Evernote was releasing a lot of upgrades. I would install the updates, go on my way, then notice that a new update was available. Eventually I took notice of the version number, and found that I had installed version 6.0.8 of Evernote ten times already, and the iMovie and XCode update several times too. So I watched to see what was happening. The App Store notifies me that there are three updates waiting to install - Evernote, iMovie and XCode. I perform the update for Evernote, the App Store announces that the update was successful, puts the details down below in the Updates Installed in the last 30 days section, the count of waiting updates drops down to 2. One minute later, the count is back up to 3, and the App Store notifies me that an update to Evernote 6.0.8 is waiting to be installed.
The recent big Mac OS X update didn't behave like this. It's iMovie, Evernote and XCode that are doing it. I've Googled for solutions but haven't got a good search term yet, because I can find no solution so far.
My older big fat white Mac Mini should still be functioning, except for the hard drive. I'm going to replace the hard drive and see if I can get it to boot up. I had that machine for my media server for years without any problems like this. If I can get it back into service, I will. I am not happy with these newer all metal Mac Minis.
I'm also going to pursue investigations into non-Apple media systems, like Plex on Linux and Rokus. Macs used to be very stable, but I haven't seen that lately.