I have two Mac Minis.
One I bought brand new last year. It's a "MacMini (Late 2012)" aka "Macmini6,1" with 4 gig RAM and a 500 gig hdd. After I got it, I had a few problems with it. Like it would greyscreen and need to be rebooted. I ascribed this to the memory drain put on it by iTunes that was trying to handle over 100,000 mp3s and 6,500 mp4s. I bought 16 gig Crucial RAM, matching it precisely to this MacMini model. The MacMini continued to greyscreen, with more frequency. I pulled the memory out. Then it would greyscreen about once a month, and as its only task was to serve those mp3s and mp4s up to my Apple TV, I tolerated that.
Then I added the Drobo. Now it greyscreens about every time I rsync anything to the Drobo, and if it doesn't greyscreen during the rsync, it will greyscreen 10 minutes later. Something is wrong.
The reports that get sent to Apple say that launchd had a problem. No cause, no other reason, not enough for me to work out what is happening. Now I suspect it might be the related to the external drives on this MacMini. The Drobo is connected directly to the MacMini. One other external USB drive is connected directly to the MacMini. Three other external USB drives are connected to a powered USB hub. All up, I have two 3T external drives and two 2T external drives and the Drobo. The goal is to move the contents of those external drives to the Drobo, eventually, when the Drobo has enough available space. But with the MacMini greyscreening so frequently, it's difficult to move the data to the Drobo.
I have read that the USB hubs can break down and cause problems. My USB hub is old. So my next attempt to fix this problem will be to replace that this weekend and see how that goes. Once I can get all the data on the Drobo, I won't need a hub as I will only have the Drobo attached.
Then I have a MacMini at work that I use for doodling with XCode and seeing if I can learn Swift and develop iPhone apps. I bought it used from a guy who used it for that very purpose. It's actually a Mac Mini Server. It's called a "Mac mini Server (Mid 2011)" aka "Macmini5,3". It's older than the Mac Mini at home that was brand new, but it's more powerful. The one at home is a Core i5, this one is a Core i7 with 8 gig RAM and two (yes two) 500 gig hdds. This one is rock solid. I have had no problems with it up till now.
Both have brand new clean installations of Yosemite. The server does not have the server software on it. That's an extra package I could buy if I wanted to, but I don't choose to.
So I had the idea that I would swap them, and take the MacMini Server home and use it for a media server, and take the MacMini to work and use it as a development machine.
First thing I did was install that 16 gig RAM in the MacMini Server. Ah, there was a conflict. The 16 gig RAM is DDR3L-1600 and the memory recommended for the MacMini Server is DDR3-1333. Different clock speed. I researched this and many people said you could run the 1600 in place of the 1333. I installed it last night, it booted just fine and it worked. I tested it with the Apple Hardware Test (boot while holding D) and it worked fine. The test ran a long time, and I left it running overnight. When I came to work this morning, the MacMini Server had greyscreened (I'm getting sick of seeing this) and the body of the computer was extremely hot. I turned it off and let it cool. When it was cool, I removed the 16 gig of DDR3L-1600 memory and replaced it with the original 8 gig DDR3-1333 memory. I turned it on, and it would go beep-beep-beep, pause, then repeat. I opened it up, reseated the memory carefully, and tried again. Success. I only hope I haven't fried that computer.
At home, I pulled the Drobo off the MacMini because I have Music Club night tomorrow night and I need the media server running. I did run the Apple Hardware Test on it, and that indicated that a sensor was failing. Not sure what that is, but I will re-run the tests this weekend and try for a clean bill of health. I am still within the first year with this MacMini so it should still be in warranty. I will check online for that. If it is under warranty, and the hardware test continues to show anything wrong, I will take it to the Apple store next week.
So that's where I am at. The MacMini Server at work is working just fine again, but I will watch it in case I have damaged it with the memory and the heat. The MacMini at home has the Drobo detached and I will try again this weekend with a brand new powered USB hub and I will run more hardware tests. If any of that fails, I will take my Mac Mini to the Apple Store and see if they can see something wrong with it.