I bought a used Mid 2011 Mac mini Server and I decided to buy two 1TB Seagate SSHD drives to replace the twin 500GB 7200RPM drives. My father has a Mid 2010 Mac mini (non-server model) with a stock 320GB 5400 RPM hard drive; so I thought I'd give him one of the 500GB 7200 RPM drives that I pulled from the Mid 2011 Mac mini Server in the process.
Having done hard drive swaps and replacements on Mid 2011 non-server model Mac minis as well as Late 2012 non-server Mac mini models, but none of the other "unibody" style Mac mini models.
That being said, given that, I was unprepared for:
a. The thermal sensors and tape on the hard drive of the Mid 2010 model (these are not present on later non-server models' drives)
b. The nature of removing the second drive of the Mid 2011 Server (and how it essentially was the last part to remove from the chassis and that to replace both drives, I basically had to disassemble the entire machine).
Transferring the hard drive films was annoying. Transferring the thermal pads/gromets/whatever was irritating as the adhesive didn't transfer and I had to finagle it so that they were in the right places at least for reassembly. It was interesting
Study both the iFixit guides and the service manuals and don't fear. The "unibody" Mac minis are really good about coming back together. But if you're not using Apple replacement parts, you'll have to transfer films and pads and that will often be annoying and/or difficult. It's a good thing to do if you have an out-of-warranty Mac mini and you want to add storage.